Hi unfortunately this is a known bug, the issue is that the time in the initrd is stuck at Nov 2019, so if you created the key that you signed your model assertion with after that date, then the image is unable to boot. You can check when your key was created with something like this (assuming that you replace NAME with the name of the key you used with snap sign -k, if you didn’t use -k, then the name is default):
snap known account-key --remote public-key-sha3-384=$(snap keys | grep -Po '^NAME\s+\K.*') | grep since
This issue is tracked in a few different bugs, mostly assigned to the foundations team / @xnox to work on at some point, I will let @xnox comment on a timeline to addressing those bugs. The bugs are: