Just for the record, temperatures much below zero are possibly detrimental for your battery. You should definetly not charge your battery at degrees below zero.
The warning above 20°C is odd. It might be that the software which triggers the warning produces an overflow or suchalike. Therefore, I would contact support and describe this as a bug, so that the developers can look into this. I would furthermore advise to reboot the phone, at room temperature, and then see if the problem persists. If not, you can test if it re-occurs if your phone got exposed to very cold temperatures again.
FTR, I doubt that a difference in temperature
is measured, as @Barbara_Ottawa suspects: this would mean at least two sensor measurements. Also, I doubt that the battery has a build-in sensor.
Also FTR, I think every battery-powered electronic device has a certain temperature range under which it is supposed to work properly - and I suppose only a tiny fraction of them will include temperatures of -10°C.