It just doesn't recognize the server's Map Frontiers, even though they are on the same version.

Version on client and the server are same. 1.20.1-2.6.0-beta.
Tested with multiple players.

Are they both fabric or forge?
Please upload the latest.log here. Also the server log if you have access to it.

the database had somehow corrupted.
deleting it solved the issue and it started working again