rockpro64 displays on monitor with HDMI port, but not on other monitor with mini HDMI port??

[frerebo]

Hi all. I have a Rockpro64 with u-boot flashed to SPI, and Armbian installed to NVMe drive*. When I use the HDMI port on the Rock to a standard HDMI port on a monitor, everything displays without issue. But recently I tried connecting to a Viewsonic va1655 using the HDMI to mini-HDMI cable, and the monitor displays fine while U-boot is doing its thing. But right before Armbian loads, the Viewsonic monitor flashes yellowish orange and then completely loses the signal. Buuut if I unplug the VS monitor and plug in the other HDMI monitor to the rock, the image is restored and Mr. Tux is patiently waiting for me to enter my password.

*I'm not sure if this matters, but I used armbian-config to install bootloader on the nvme: