So I had started this previous thread (link below) before I realized the depth/breath of this issue.
I have realized that my 965 has not been recording temperature on the runs and rides I have done where the Tempe sensor I have stored in my sensor library is not present. On the rides I have done where the Tempe was present at the start (i.e. it was in my garage), it records the initial temperature and then holds that for the rest of the ride.
Note that temperature recording works fine when the Tempe is present.
However, on my cycling rides I typically don't bring the Tempe along, the watch mounted on the handlebars will record the temperature just as well as an external Tempe would (as it's not impacted by my skin temperature).
Compounding this issue, in the Activity settings (either Run or bike) when you scroll down to the "record temperature" option, it is greyed out and toggled permanently ON (i.e. you can't toggle it off) and the Tempe sensor last used shows the device name underneath.
This is different behavior than my previous 945 displayed where it would (sensibly) record temperature from the Tempe when present, but from the watch's internal sensor when not.