More news, this is not u-boot issue. I git cloned current armbian build from github.com and compiled kernel+bootloader. This resulted in `output/debs/linux-u-boot-current-rock64_21.11.0-trunk_arm64.deb` I inspected this file and can see there is no `preinst` or `postinst` files so manual update of the bootloader is necessary (without doing more search). Looking at the `/usr/lib/u-boot/platform_install.sh` script and the files in the *.deb I can see the relevant dd commands that are needed to update the bootloader.
With `Armbian_5.91_Rock64_Debian_buster_default_4.4.184_desktop.img` as the image written to SD card, I wrote the newly compiled bootloader and I restarted the rock64. It did not display any HDMI/video artifacts.
To further exclude u-boot being problematic, I wrote `Armbian_21.08.1_Rock64_bullseye_current_5.10.60.img` to SD card then extracted the bootloaded from Armbian 5.91 and wrote it to SD card. This did not fix the HDMI artifacts.
Now time to investigate which package is causing the issue This will probably take a long time.