Not a minor sync bug

This is about the iOS Joplin app, v 10.3.1 in conjunction with desktop app v 1.3.18:
desktop app works flawlessly across several Macs. iOS app seemed to sync well (never made changes on the iOS side). But after a couple of syncs, with changes on the desktop side, it started failing to sync. Changes are not imported, but there is no error messages, no conflicts, no other indication. It simply doesn't notice that sth has changed on the server.
Since I did not upgrade either app before or after the problem occured, I think this is not covered by the similar report created 21.8. and the answer by laurent. But I can't be sure of course.

Tried deleting the app from iOS, clean slate, start all over again. Same result. This time I noticed the last line in the error log: "Could not upgrade to database v34". Funny thing is, the config page inside the iOS app says: Database v34.