S9+ apps dropping out of full screen periodically after phone update

[Seadn]

Two updates ago on my s9+, when watching videos - Netflix, YouTube, what have you - in horizontal fullscreen mode, after 5-10minutes it falls out of fullscreen. With YouTube, it rotates back to vertical small-window, with apps like Netflix it sends me to the homescreen with a pop-out window of the video that continues to play. Restarting the phone did nothing. Reinstalling apps did nothing. Installing the most recent update today changed nothing. I wasn't able to revert to an earlier version of the phone - it said no because of security reasons. Its driving me insane - anyone experience this/know what's going on?