We have a couple Bullets in use, and quite a few Bridges. I don't think we have seen this issue. I know this year we have seen some 15F temps. And last year we had a few 0's (I think was the coldest) And hadn't seen this.
I remember having issues like this with our older Deliberants and such for customers. Almost all of our power boxes are outdoor, without any insulation. Some just have 2 POEs chained together. So there isn't any heat to speak of in that box. I just find it pretty hard to believe a bullet would freeze. Ever held one of those on the bench? They get pretty darned warm even at idle. And of course they are "rated" at -40C
Of course after saying this, mine will magically start to do the same thing
I have heard in the past that as ambient temps drop, devices require more power (current) to operate. I can't recall why, or if it's even true. But, if there is any kind of issue with cabling, POE, or crimps, it will show it's ugly head at the worst time. If it is true, your wraps might be helping in the form of the radio needing less current since it's warmer. It would also correlate to the lowering of power output solution.--