It's not a bug. It's designed that way to prevent any abuse of the epics.
If members have left the fleet by themselves or the members have been kicked out by the fleet captain, the contributions won't reset. To get the correct award from an epic, it is important to team up with people who can finish the epic to the end.

If you don't want people you don't know to join your fleet, you can turn on "Invite Only" in the fleet settings to only allow your friends to join your fleet.