I have this problem too. It started a month ago and usually went away by turning the phone on and off, but it now happens with such frequency that I've given up and will take it back to the store. I initially thought be to wait for an OSI update which might fix the problem (which seems to be software issue if it can be fixed by turning the phone off and on), but I don't want to risk being stuck with a lemon 12 months later. Apart from this problem, my camera frequently freezes whilst I take photos/videos, particularly using portrait mode - which is very annoying given that the phone costs S$1800 (I think it is currently the most expensive phone on the market).
I first called apple support who ran the diagnostics, but nothing came up. Said phone/camera is working fine, but clearly it is not. My purpose of the call as to see if they have encountered similar issues from other Iphone X owners and whether it is worth taking it down to the apple service center or wait for an OSI update. To my surprise the tech guy answered no to the first question and was a bit clueless as to the second. He didn't really know what to do apart from suggesting that I run the diagnostics and do a full phone reset (which I am hesitant to do because it takes ages to back-up then reboot and reinstall)... After i suggested that the message on the phone said I should take it to the Apple Store, he made an appointment for me.......
Will let you know how it all goes.
I am a bit disappointed in apple's reliability.