Sm-R800 continuously powers down due to "overheating"

Sm-R800 Galaxy watch continuously powers down ~20 seconds after powering on due to "overheating" - but the watch is a stable 70°f and definitely not overheating. Any ideas? I've currently:

Ensured the watch and app software are up to date. No dice

Factory reset the watch. No dice

Left the watch in a bag of rice for 72 hours. No dice

Swapped out the charging cable and the charging station/dock. No dice

I'm more than happy to fix it myself, I just don't know what I should be looking to replace.

Many thanks guys!