The logic is a little tricky to get right because there's a set sequence of steps that needs to happen in order for the group sync to succeed.

For example, between NodeBB and Lemmy:

1. NodeBB admin tries to follow a Lemmy community
2. NodeBB marks the follow as pending
3. Lemmy records NodeBB as a follower
4. Lemmy community tried to send an Accept back
5. NodeBB marks the follow as established.

If steps 1-3 succeed but 4-5 fail, then posts might be sent to NodeBB, but NodeBB will silently drop them because they're not addressed to anyone it thinks is following the sender.

@freamon thanks for the heads up about lemmy's accept shenanigans, I'll take that into account... or maybe ask Felix about it.

@arachnibot @FrankM