I have same problem. But it started when I use guest option the first time, one month ago, then soft keys didn't go anymore while they are in lower side of movile, if I turn movile and buttoms go to upper side (under sony logo), it works right. Yesterday, I had an other problem about movile auto turns off with high battery and I returned to fabric devices and the return button problem followed not working. I tried Nova Launcher solution, but I found it didn't work. After long time searching for solution I found in play store an app named: HANDY SOFT KEYS, it puts to right side a menu with return, home and apps buttoms, only needs to give permision on accesibility. When I need give some permisions to order apps (because I have to configuration all an other time) I need disable it for a few time, but it is a little problem compare to don't have return buttom. I hope this app help us while Sony don't give a good support. After reading this forum I don't think that I send my mobile although I am under warranty. Also say that the oder probem with auto shutdown has not been solved either.
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