I have the same.
iPhone 12 mini. Restores backup from iPhone 8. Everything is working ok. But no notifications on some of the apps.
For instance Instagram. Reinstall, logout/in, reset password instagram, reset network, reset settings, reinstall backup, tried without backup, installed the 14.3 beta to see if that makes a difference.
Nothing seem to work. iCloud is working for me. Mail, iMessage, Whatsapp, health app seem to be working ok with notifications. Some other apps don't...
I have no clue. If someone has.
EDIT:
I reset the device and manually set it up. Some 2 hours later, I think I have everything back in place.
Half way into the process, the apps where notifications were missing, started to show notifications again.
Apparently you can't use a backup that you updated since 2008 with the first iPhone... weird aint...