I'm also having trouble with this: the navigation bar pops up to stay, even though I tell it to hide. I have tried to tell it to hide in many different ways too, by "Always", and by any touch... Can't remember all of the ways I've tried atm, but have been tinkering around with this a few days now - testing to "dim navigation bar" instead, testing to combine "dim.." and "hide.." but instead the navigation bar either covers the bottom of my game(game-breaker in my case), or squeezes (resizes) the screen from bottom up for the space of the navigation bar(done with using only "dim..", which is the best solution for now, but it is a bit annoying for my game-play to have those buttons available during the play).
What I have noticed while testing is that the navigation bar does hide, but as soon as I touch anywhere on the screen it shows up again. It is as if the first touch after that the navigation bar has hidden, is telling it to show itself again..
When testing, I'm exporting the game as an Android version 7.1 APK with API 25, running the game on a Sony Xperia XZ Premium in Android v. 9.
PS: Noticed that sometimes I can work my way around the issue in-game, by having the "Resume" button of my pause screen attempting to "hide the navigation bar", which will only work until next touch. However, sometimes when I tab out of the window - leave the app in the background and return to it, this can remove the navigation bar.