While gathering cats, i noticed that if one player collects some, then logs off, followed by an other player gathering cats, the advancement only gets synced between them when both log in at the same time.

This seems like it is a fixable thing, by updating the achievements for a player on login, based on the data for every other player in the advancement/uuid.json, instead of based on the data for each currently online player.

I haven't investigated thoroughly though, so i might be mistaken. But the code also seems to suggest this behavior.

I use CooperativeAdvancements-1.21.4-fabric-1.4.2.jar