Hi all!
This has been driving me crazy for several hours, but I have managed to identify the problem and a workaround that I think might be useful for others.

There is the following issue with tags and syncing that is crucial if people want to use tags to work collaboratively (for example for inclusion and exclusion criteria in a systematic review!):

Person A and Person B each tag the same article (with different tags). Person A syncs, then P(B) syncs (and now has both tags), but when P(A) syncs again, they won't get P(B)'s tag.
It seems like Zotero doesn't realize that it has to re-sync those elements when P(B) does their syncing. I assume that it just keeps a log of items that change after each sync, and since P(B) had added their tag before the sync, Zotero doesn't think it has to sync that element.

An easy work-around is as follows: after P(B) has synced for the first time and gotten both tags, they can add a new tag (called for example 'syncme!') to nudge Zotero to sync that element again. Now, when P(A) syncs again it will get both tags (plus the 'syncme!' tag).

I have been searching for a while in this forum without finding the solution to my problem, so now that I found out what's happening and a way around, I think it might me useful to share it with the community (and the developers might address it).
Take care!