I noticed that too but I'm unsure of the technicalities.

At the moment I'm guessing that either comments just don't sync back or that what we see here is only a clone of this post from lemmyworld on here and we're commenting on the clone instead of the original post on their instance.
Those are only wild guesses though since I'm not too familiar with how kbin works but that would be why I personally think this could be.

Anyways the devs are/will be probably working on this too since some other threads have adressed problems with commenting to other instances and or replying to comments of this instance.