There was a post by u/kafunshou on Reddit reporting this issue: by https://www.reddit.com/r/Anki/comments/r3d2e4/full_sync_required_without_comprehensible_reason/
I and other people also experience this issue, so I thought I’d double it here.
The original post’s text:
"On both my iOS devices, an iPhone and an iPad, while trying to sync I get the message that the decks were significantely changed so a full sync is required.
I synced without any problem from my iPad last night (after the time where the day switches for Anki) and neither did I use Anki anywhere else since then nor did I change anything on a deck that would require such a sync. I also never had something like that in the three years I’m using Anki now. All required full syncs had a comprehensible reason (updated scheduler, changes on decks etc.). But this one from today doesn’t.
Does anybody here has that problem too? Is there something wrong with the servers?
I’m not sure whether uploading everything from my iPad (which has the newest status as I synced from it last) would do damage as the behaviour doesn’t make any sense.
I use the newest versions of Anki on desktop (Windows 10 and Linux) and on iOS. I didn’t install any updates for maybe two weeks. I also disabled auto-updates for Anki on all platforms. Scheduler is V2."