How to remove virtual buttons on android?

Question

Android

Model: Asus zc520tl (ZenPhone 3)
the essence of the problem:
it does not have hardware buttons (home, back). There are virtual ones that automatically hide when you start applications (games, videos, books). After a while they stopped disappearing. In some applications they disappear, in others they don't. How to fix it? full reset did not help.