I have only noticed it on 18.1. I reported it on both devices via the Feedback option. I haven't paid attention enough to know what triggered it, but based on the reports about it being the hand detection thing to prevent accidental touches, if I use my phone in a way to make sure none of my hand touches the screen in any way, I don't seem to get it.Sorry to hear that. So it is beta specific then? I updated immediately after setup, so never got to see how vanilla 18.0 worked on the 16PM.
That said, after my reboot, I am holding my phone in my right hand with my palm lightly touching a tenth of an inch or so of bezel and no touch issues. Too early to tell, but it seems okay so far.
I wonder if it's the thinner bezel with that feature that makes it act up. You hand is just close enough to the screen that it registers and locks out accidental swipes. Not sure.