I have a similar issue. I had to reinstall penultimate after formatting my iPad Pro (15.3.1), and the notes never finished syncing. A few notes and notebooks are readable. I can see all the others in the app, as icons with the title (but without preview), but if try to open them a message shows, saying "We'll load this note once your account has finished syncing". Syncing again manually does not solve the issue.
Marco.