Originally Posted by Link_of_Hyrule
I found out that the custom lock screen is what was causing on screen navigation buttons to disappear. As far as I can tell it's any lock screen replacement that will cause the problem.
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Try uninstalling gmail, then the lockscreen. The nav bar should still be there. I had a similar issue with an odd fix. It seems like most people agree on gmail notifications causing the issue....for some reason. I think somewhere in gmail you can disable notifications and that would work, but I remember having issues getting into gmail. I think re-installing gmail made it work, but I'm not positive. I'd prefer using a new lockscreen, and the browser for gmail though :P