Report of the same issue in lemmy support - https://hackertalks.com/post/9701951

If a user is the only subscriber on a instance, and gets banned, and unbanned, then unban wont federate.

If lemmy-federate would subscribe even if there are already local subscribers, then this lemmy issue would be moot for lemmy-federated communities (there would always be at least one subscriber on the instance with the lemmy-federate bot, so the unban gets federated).