Ok, so this morning I tried 4k/60fps recording in safe mode, cover off, screen brightness as low as it could go, dark room, no other apps running, ambient temperature in the low 20s celcius.
Recording stopped after 8m18s. Longest one yet. Wooooo! :-/
I've reported the issue to google pixel support chat, which was escalated to a "specialist", who sent me a very unhelpful long-winded cut/paste that boiled down to "phones get hot sometimes. try using it in cooler/easier conditions".
Maybe there are just a couple of dud units, but if you live in a warm climate I'd think twice about purchasing a pixel 5 for a little while at least. Yes, 4k/60fps recording is not light-weight, but my wife's galaxy fe20 handles it easily, and the car mount GMaps shutdown is unsettling.