Do this, put you phone on the charger, turn it to landscape and then start Waze. When it first starts and shifts into landscape, it will still be portrait sized. This same thing happened in early iOS 10 betas too and about beta 3, it was corrected.
I also have my phone in zoomed mode so this also may have something to do with the issue. The more annoying one is the wallpaper being messed up all the time after I exit.
**EDIT**
It looks as though the wallpaper gets stuck in landscape mode after exiting other apps that go into landscape mode. I was using Fox Sports Go to watch a ball game. This app forces landscape mode during video. When I exited the app, my wallpaper was stuck in landscape mode again.