That's weird behavior and I don't have that on my GOS device. Maybe you have a hardware defect (the proximity sensor for calls comes to mind, but that should only turn off the screen while using the phone app).
Anyway, to isolate the issue, could you start a fresh user profile and see if, without changing the settings, the odd behavior remains? If not, probably some setting or app on your main profile is causing it...
Also, do you have developer settings enabled? If yes, disable them (which resets all the changes you did there) and see if that fixes your issue.
Just my random ideas, hope they help.