Thank you for this post, I am glad I found it while troubleshooting my issue.
I was also able to get it to work on 12.2.1, I added "video=HDMI-A-1:720x576M@50D" to the config txt file and was able to get output to display on the Sienna screen.
Based on the previous comments I was hoping changing the refresh rate "video=HDMI-A-1:1280x720M@50D" would work but it did not, the screen went black after the rainbow screen.
Anyway, I thought I would share since this post helped me a lot.