I have found an issue which occurs if a unit is blacklisted, then later is un-blacklisted, where it will say that the group "needn't share" to other groups. This also occurs when a group is added mid-game, while the other units are already engaged in combat.

@Alexander
The debugging mesage "xxx needn't share somebody to xxx" means that the receiver group has already known the shared target, and their knowledge about this target is not lower than the sender's knowledge.