I have a similar issue with my Apple Watch Series 7 since upgrading to watchOS 10.0. Had contacted Apple Support who could not solve it either. However, on pairing the watch with a different iPhone connected to a different iCloud account solved the issue.
Prima facie, it appears to have something to do with my Apple Watch backups on iCloud but don’t know the root cause yet or the resolution. Deleting some old Apple Watch backups from iCloud had temporarily caused some of the new watch faces to reappear and since then it has been a game of hide and seek with watch faces for me.