You either use Maps in PORTRAIT or LANDSCAPE mode.
It’s a very polarizing issue, but one thing is universal: when the phone changes to your non-preferred orientation, it’s frustrating — and frustrating at the worst possible time: when you’re wondering if you should turn left or right, or keep driving straight.
If a reading app like Pocket can implement (app-specific) rotation lock, there’s nothing keeping large companies like Apple and Google from doing so too.
I for one would toggle to the portrait-mode only setting (if it existed) and never think about it again, ever. It would be an easy thing to add in the app.