Blowing up the picture seems to show possibly a scab of armor (lower right of hole) came off?
I don't think the cold weather during the Battle of the Bulge was
that extreme. It was 'cold' because of the low temps, combined with humidity and winds. Basically miserable weather that did not let frost to develop.
I am not sure I agree that an internal fire will make a gun leave battery. It is mechanically locked at that point. It is not being 'locked' by the recoil mechanism. I
could see the scenario of a internal fire possibly disabling the recoil mechanism hydraulics/pneumatic, and then a round in the chamber cooking off, and
then the gun could not return to battery.
Some reports say that Germans would just open the hydraulics, leaving the gun as is. The thought being they could retrieve it. Firing the main gun in this condition does a lot of damage.