I found this thread from last year with a similar issue to yours 2025-08-28 - A particular note is no longer synchronising - #34 by Coffee, which is implied was also remedied by export as MD and FrontMatter and re-import (with further remediation required). The OP was also a previous Joplin Cloud user and used notebook shares.
Did you ever use any notebook shares while you were using Joplin Cloud?