I tested it on Studienka (where it actually happened) but I could not replicate the bug. At the time it happened the view was so weird it looked like the tank had a rear MG. But you are right, Panthers don´t have such thing.
If I ever see it again I will take a screenshot. These were the facts:
Map: Studienka
Tank: Panther A
Position: hull MG
Situation: I spawned on my SL who was driving the tank. The hull MG did not show optics, though the gun was movable with the mouse. At a certain angle the soldier´s hands and hand-held MG42 kit became visibile in a buggy odd way.