After testing it with verbosity=7 on both vendor and current, there's nothing suspicious in the kernel logs -- on vendor, I get:
root@radxa-zero3:~# journalctl -k -b 0 | grep -iE 'hdmi|drm|gpu|edid|cec'
Apr 08 11:26:19 radxa-zero3 kernel: OF: fdt: Reserved memory: failed to reserve memory for node 'drm-logo@0': base 0x0000000000000000, size 0 MiB
Apr 08 11:26:19 radxa-zero3 kernel: OF: fdt: Reserved memory: failed to reserve memory for node 'drm-cubic-lut@0': base 0x0000000000000000, size 0 MiB
Apr 08 11:26:19 radxa-zero3 kernel: platform fe040000.vop: Fixed dependency cycle(s) with /hdmi@fe0a0000
Apr 08 11:26:19 radxa-zero3 kernel: platform fe0a0000.hdmi: Fixed dependency cycle(s) with /vop@fe040000
Apr 08 11:26:19 radxa-zero3 kernel: [drm] all windows should be assigned, full plane mask: Cluster0 | Cluster1 | Esmart0 | Esmart1 | Smart0 | Smart1[0x3f], current plane mask: [0x0
Apr 08 11:26:19 radxa-zero3 kernel: [drm] use default plane mask
Apr 08 11:26:19 radxa-zero3 kernel: rockchip-vop2 fe040000.vop: [drm:vop2_bind] vp0 assign plane mask: Cluster0 | Cluster1 | Esmart0 | Esmart1 | Smart0 | Smart1[0x3f], primary plane phy id: Smart0[4]
Apr 08 11:26:19 radxa-zero3 kernel: rockchip-vop2 fe040000.vop: [drm:vop2_bind] vp1 assign plane mask: [0x0], primary plane phy id: INVALID[-1]
Apr 08 11:26:19 radxa-zero3 kernel: rockchip-vop2 fe040000.vop: [drm:vop2_bind] vp2 assign plane mask: [0x0], primary plane phy id: INVALID[-1]
Apr 08 11:26:19 radxa-zero3 kernel: [drm] failed to init overlay plane Cluster0-win1
Apr 08 11:26:19 radxa-zero3 kernel: rockchip-drm display-subsystem: bound fe040000.vop (ops 0xffff80000939a0c0)
Apr 08 11:26:19 radxa-zero3 kernel: dwhdmi-rockchip fe0a0000.hdmi: Looking up avdd-0v9-supply from device tree
Apr 08 11:26:19 radxa-zero3 kernel: dwhdmi-rockchip fe0a0000.hdmi: Looking up avdd-0v9-supply property in node /hdmi@fe0a0000 failed
Apr 08 11:26:19 radxa-zero3 kernel: dwhdmi-rockchip fe0a0000.hdmi: Looking up avdd-1v8-supply from device tree
Apr 08 11:26:19 radxa-zero3 kernel: dwhdmi-rockchip fe0a0000.hdmi: Looking up avdd-1v8-supply property in node /hdmi@fe0a0000 failed
Apr 08 11:26:19 radxa-zero3 kernel: dwhdmi-rockchip fe0a0000.hdmi: Detected HDMI TX controller v2.11a with HDCP (DWC HDMI 2.0 TX PHY)
Apr 08 11:26:19 radxa-zero3 kernel: dwhdmi-rockchip fe0a0000.hdmi: registered DesignWare HDMI I2C bus driver
Apr 08 11:26:19 radxa-zero3 kernel: dwhdmi-rockchip fe0a0000.hdmi: error -ENXIO: IRQ index 1 not found
Apr 08 11:26:19 radxa-zero3 kernel: rockchip-drm display-subsystem: bound fe0a0000.hdmi (ops 0xffff8000093b2958)
Apr 08 11:26:19 radxa-zero3 kernel: [drm] Initialized rockchip 4.0.0 20140818 for display-subsystem on minor 0
Apr 08 11:26:19 radxa-zero3 kernel: rockchip-drm display-subsystem: failed to parse loader memory
Apr 08 11:26:19 radxa-zero3 kernel: EDID block 0 is all zeroes
Apr 08 11:26:19 radxa-zero3 kernel: EDID block 0 is all zeroes
Apr 08 11:26:19 radxa-zero3 kernel: dwhdmi-rockchip fe0a0000.hdmi: failed to get edid
Apr 08 11:26:19 radxa-zero3 kernel: EDID block 0 is all zeroes
Apr 08 11:26:19 radxa-zero3 kernel: dwhdmi-rockchip fe0a0000.hdmi: failed to get edid
Apr 08 11:26:19 radxa-zero3 kernel: rockchip-vop2 fe040000.vop: [drm:vop2_crtc_atomic_enable] Update mode to 1280x720p60, type: 11(if:HDMI0, flag:0x0) for vp0 dclk: 74250000
Apr 08 11:26:19 radxa-zero3 kernel: rockchip-vop2 fe040000.vop: [drm:vop2_crtc_atomic_enable] set dclk_vop0 to 74250000, get 74250000
Apr 08 11:26:19 radxa-zero3 kernel: EDID block 0 is all zeroes
Apr 08 11:26:19 radxa-zero3 kernel: rockchip-drm display-subsystem: [drm] fb0: rockchipdrmfb frame buffer device
Apr 08 11:26:19 radxa-zero3 kernel: rockchip-drm display-subsystem: [drm] run display error_event monitor
Apr 08 11:26:19 radxa-zero3 kernel: Registered IR keymap rc-cec
Apr 08 11:26:19 radxa-zero3 kernel: rc rc0: dw_hdmi as /devices/platform/fe0a0000.hdmi/rc/rc0
Apr 08 11:26:19 radxa-zero3 kernel: input: dw_hdmi as /devices/platform/fe0a0000.hdmi/rc/rc0/input0
Apr 08 11:26:19 radxa-zero3 kernel: input: hdmi_cec_key as /devices/platform/fe0a0000.hdmi/dw-hdmi-cec.1.auto/input/input1
Apr 08 11:26:19 radxa-zero3 kernel: vdd_gpu: supplied by vcc_sys
Apr 08 11:26:19 radxa-zero3 kernel: vdd_gpu: 500 <--> 1350 mV at 900 mV, enabled
Apr 08 11:26:19 radxa-zero3 kernel: rockchip-pm-domain fdd90000.power-management:power-controller: Looking up gpu-supply from device tree
Apr 08 11:26:19 radxa-zero3 kernel: rockchip-pm-domain fdd90000.power-management:power-controller: Looking up gpu-supply property in node /power-management@fdd90000/power-controller failed
Apr 08 11:26:19 radxa-zero3 kernel: mali fde60000.gpu: Kernel DDK version g25p0-00eac0
Apr 08 11:26:19 radxa-zero3 kernel: mali fde60000.gpu: error -ENXIO: IRQ JOB not found
Apr 08 11:26:19 radxa-zero3 kernel: mali fde60000.gpu: error -ENXIO: IRQ MMU not found
Apr 08 11:26:19 radxa-zero3 kernel: mali fde60000.gpu: error -ENXIO: IRQ GPU not found
Apr 08 11:26:19 radxa-zero3 kernel: mali fde60000.gpu: Insufficient register space, will override to the required size
Apr 08 11:26:19 radxa-zero3 kernel: mali fde60000.gpu: Register window unavailable
Apr 08 11:26:19 radxa-zero3 kernel: mali fde60000.gpu: Failed to map registers
Apr 08 11:26:19 radxa-zero3 kernel: mali fde60000.gpu: Register map failed error = -5
Apr 08 11:26:19 radxa-zero3 kernel: mali fde60000.gpu: Device initialization failed
Apr 08 11:26:19 radxa-zero3 kernel: mali: probe of fde60000.gpu failed with error -5
Apr 08 11:26:19 radxa-zero3 kernel: EDID block 0 is all zeroes
Apr 08 11:26:19 radxa-zero3 kernel: #0: rockchip-hdmi0
Apr 08 11:26:19 radxa-zero3 systemd[1]: Starting modprobe@drm.service - Load Kernel Module drm...
Apr 08 11:26:19 radxa-zero3 systemd[1]: modprobe@drm.service: Deactivated successfully.
Apr 08 11:26:19 radxa-zero3 systemd[1]: Finished modprobe@drm.service - Load Kernel Module drm.
Apr 08 11:26:19 radxa-zero3 kernel: panfrost fde60000.gpu: clock rate = 594000000
Apr 08 11:26:19 radxa-zero3 kernel: panfrost fde60000.gpu: bus_clock rate = 500000000
Apr 08 11:26:19 radxa-zero3 kernel: panfrost fde60000.gpu: Looking up mali-supply from device tree
Apr 08 11:26:19 radxa-zero3 kernel: panfrost fde60000.gpu: mali-g52 id 0x7402 major 0x1 minor 0x0 status 0x0
Apr 08 11:26:19 radxa-zero3 kernel: panfrost fde60000.gpu: features: 00000000,00000cf7, issues: 00000000,00000400
Apr 08 11:26:19 radxa-zero3 kernel: panfrost fde60000.gpu: Features: L2:0x07110206 Shader:0x00000002 Tiler:0x00000209 Mem:0x1 MMU:0x00002823 AS:0xff JS:0x7
Apr 08 11:26:19 radxa-zero3 kernel: panfrost fde60000.gpu: shader_present=0x1 l2_present=0x1
Apr 08 11:26:19 radxa-zero3 kernel: [drm] Initialized panfrost 1.2.0 20180908 for fde60000.gpu on minor 1
Apr 08 11:29:48 radxa-zero3 kernel: EDID block 0 is all zeroes
Apr 08 11:29:48 radxa-zero3 kernel: dwhdmi-rockchip fe0a0000.hdmi: failed to get edid
And on the failing one, just the same old:
Apr 09 22:04:32 radxa-zero3 kernel: /vop@fe040000: Fixed dependency cycle(s) with /hdmi@fe0a0000
Apr 09 22:04:32 radxa-zero3 kernel: /hdmi@fe0a0000: Fixed dependency cycle(s) with /vop@fe040000
Apr 09 22:04:32 radxa-zero3 kernel: /hdmi@fe0a0000: Fixed dependency cycle(s) with /hdmi-con
Apr 09 22:04:32 radxa-zero3 kernel: /hdmi-con: Fixed dependency cycle(s) with /hdmi@fe0a0000
Apr 09 22:04:32 radxa-zero3 kernel: rockchip-drm display-subsystem: bound fe040000.vop (ops vop2_component_ops [rockchipdrm])
Apr 09 22:04:32 radxa-zero3 kernel: dwhdmi-rockchip fe0a0000.hdmi: Detected HDMI TX controller v2.11a with HDCP (DWC HDMI 2.0 TX PHY)
Apr 09 22:04:32 radxa-zero3 kernel: dwhdmi-rockchip fe0a0000.hdmi: registered DesignWare HDMI I2C bus driver
Apr 09 22:04:32 radxa-zero3 kernel: rockchip-drm display-subsystem: bound fe0a0000.hdmi (ops dw_hdmi_rockchip_ops [rockchipdrm])
Apr 09 22:04:32 radxa-zero3 kernel: [drm] Initialized rockchip 1.0.0 for display-subsystem on minor 0
Apr 09 22:04:32 radxa-zero3 kernel: rockchip-drm display-subsystem: [drm] Cannot find any crtc or sizes
Apr 09 22:04:32 radxa-zero3 kernel: rockchip-drm display-subsystem: [drm] Cannot find any crtc or sizes
So I suspect there's some EDID problem. (I'll try forcing EDID next, but I have a few more boards ordered to see if it's a hardware problem or not (or is this a sneaky way of making me buy more CAD$60 boards...)