As described by someone here:
https://bugtracker.arcengames.com/view.php?id=19593 this bug causes assault transports to retain their large and powerful shot volley, after all ships are unloaded. This persists from planet to planet, but not across saves. Saving and reloading will fix the specific ship(s).
It also seems that Assault Transports may not be firing their full volley at full capacity, as visually at least they seem to fire only 14 of 20 potential shots at full capacity.
