Is this likely somehow related to the cold temp?

Yep, most definitely. The older non-solid-state transponders use a
cavity in their tx circuit, and the geometry of the assembly is
critical to getting it to work. It can be close enough that it works
when it is warm and expanded to its usual operating conditions, but far
enough off that it won't work when it's cold. My KT-76A did this a
couple of times, and needed to be tweaked back in by the radio shop.
It's an easy enough fix, but they do tend to slip out of alignment
again after a while.