I have been having the same problem since I put in my beta and was just looking for information. I have also installed the latest public beta in series 5 and am experiencing the random disappearance of FACEs. I believe it is a glitch in the OS as I can swipe up and down to see the control center and notifications. I also feel that this version is very buggy, as the overall response is sluggish and the wallet display is not responsive.

watchOS 9.1 Public Beta 3 has released, so I have installed it on my Apple Watch. Unfortunately, however, the problem does not seem to have been solved. I have only tried InfoGraph, but there is a slight visible lag in transitioning to always-on mode when covered by hand, unlike faces which obviously has no other problems. Also, if left in place for a while, it takes a few seconds to touch the screen to return to it, and the response is also very slow. The workaround for the time being is to use another stable alternative faces, as was posted earlier. In my case, Solar and the Photo face are stable and usable.

In the newly released 9.1 beta 4, this problem seems to have been fixed. It seems InfoGraph is now stable.