In navigation bar sometimes only back button is visible. When that happens I also can't pull down notifications and clock is not visible. While "tap to wake" option under display settings is helping me to get this problem less often it doesn't solve it.
It happened about half an hour ago again and I was going everywhere trough settings and it came back to normal functioning when I enabled system UI demo mode under developer options. You can find it under system > advanced > developer options > system UI demo mode.
There are two options. Enable demo mode and show demo mode. Everything came back to normal when I switched on second option. But of course you can't leave it on, so for now I left only first option on to see will it for some reason stop this issue.
It is possible this doesn't have anything to do with bug, maybe it just came back to normal and I was at that moment at that place in settings. I'm sharing this so if someone is willing to try also.
If someone doesn't know how to activate developer options, settings > about phone > build number (click it many times (5 or 10?))
I hope this works and helps.