We were wondering why our on-board GoPro hadn't worked...
...and that might be the problem. Our hull was SO effective at insulating our gear that we built up a massive heat reservoir. The GoPro likely got even hotter than the peak of 37°C, as it was nestled among the battery banks, and the GoPro itself puts out a good deal of heat.
We'd spent so much time worrying about keeping things warm that we didn't consider about keeping them cool. This was exacerbated by the fact that our balloon prep took longer than expected and by the fact that we got our payload flight-ready much more quickly than expected.
Something we can test!
(N.B. for those less metrically inclined: 37 ° Celsius is about 99° Fahrenheit. ~JMH)