FYI, I was having trouble upgrading to the 11.3 Big Sur beta. I tried everyting (unenrolling and re-enrolling, rebooting, etc).
These commands fixed it:
sudo /System/Library/PrivateFrameworks/Seeding.framework/Versions/A/Resources/seedutil unenroll sudo /System/Library/PrivateFrameworks/Seeding.framework/Versions/A/Resources/seedutil enroll DeveloperSeed
After that, 11.3 beta immediately showed up in Software Update.
Oddly, I had no trouble at all on my Intel mac - only on the M1 did I hit this problem.