The sync between instances is all over the place right now. The sync doesn't seem to work properly between different versions of Lemmy. Some instances (like lemmy.world) are already on 0.18.1, while most others are still on 0.17.4. There also seem to be severe sync issues between Lemmy and kbin. I really hope this will work better soon, it's a really bad timing for issues like this.
I had the same issue, newer posts however should work fine, it's just the old posts before you subscribed don't seem to fetch the comments properly. I hope it will get sorted out soon.