Yeah, as mentioned - Lemmy is often quite bad at sending the 'Accept' back for a Follow. It's particularly the case if the instance hasn't heard of you before, because in the process of fetching your user and instance details, it 'forgets' about the Accept. Within Lemmy, everyone's sort of got into the habit of 'Subscribe / Unsubscribe / Subscribe'. (where the first 'subscribe' lets them get your details, and the second one actually gets an 'Accept' back.)
PieFed is more reliable at sending 'Accept' back, so it's maybe useful to check your logic from this end with a community from there, e.g. https://piefed.social/c/antisocialmedia
Looks like I was wrong before: whilst it's true that Lemmy isn't always great at sending an 'Accept' back, that's not the issue here. I was thinking about it from the perspective of a Person following a Group, not a Group following a Group.
It'll be interesting to see how the Lemmy devs respond. It's not really relevant for PieFed, because I'm not sure that there's communities there that have enough activity to warrant syncing to a category, but I thought about what would happen if you tried, and it would fail. The follower would be checked to see if they are a Person, and the request would go no further if not (there's no handy error returned, though).
I'm not sure it would be possible to enable, with the way the database is currently set up. We have a 'community_member' table, that looks like:
So if the 'user_id' was actually a Group instead of a Person, then it would have a join to the wrong table.