Ubuntu 20.04 on a Raspberry PI 2B+ 1.2 fails to pass the rainbow screen, works perfectly with Raspberry PI OS
I’m not sure if you managed to fix this since this was posted, but I had the exact same issue.
I managed to fix it by switching to a beefier power supply (my 61-watt one instead of my 5-watt one I was using).
Hope this helps.