I imagine it has something to do with sparse tracking. The final ascent / decent is I think the better part of an entire day. You can't have full high accuracy GPS on for that long and have the battery last. So you may put the device away for a while, and get it back out when you finally summit to track that you were there. And lower on the mountain you might have extra ridges in the way, like in a city with lots of tall buildings, where you just can't get the best signal. And on Everest there is no WiFi or cell signal to cheat with. I would also think that personal device GPS may get a bit confused at that altitude. That is not really the expected working conditions.

Phone off because it's not useful while climbing, but then gotta whip it out for that selfie at the top?

In my experience, GPS doesn't work well when the device is in a pack or even a thick jacket. Also, batteries don't work well in the cold. Combine those two things, it's hard to get continuous GPS tracking in cold environments.

'battery saving" location mode uses cellular signals to triangulate your position. I've seen this happen before on the map in gFit app I guess when gps wouldn't lock

GPS spoofing? Device can't track on the way up? I'd like to know too.