after some research, I determined this specifically happens if you activate apple maps navigation via Siri. if you just open maps regularly then the screen will stay on as expected. but that's kind of absurd; the main use case for navigation is driving, which is exactly the hands free environment where you'd want to be able to use Siri. after googling around, supposedly this issue has been around at least since iOS 11 (a release I'd skipped entirely). so I'm just curious if this happens to anyone who opens maps via Siri, or if its an isolated bug that some sort of settings reset might fix. I don't want to reset for the heck of it, because reconfiguring stuff is a pain, and apple has let their software slide to a state of bugginess where you could easily be told to "try resetting" every couple days.