This is happening to me as well, but I'm still on High Sierra 10.13.3, so it's not strictly a Mojave thing. As @Tom Klaber mentioned, it's not technically random, but it is definitely wrong, and very disconcerting.   I know this thread is about Mojave, so I don't want to go to deep here, but another issue I've experienced is: If I render a camera viewport on a sheet layer it will render correctly the first time, but when I switch to a different sheet, then return to the sheet just rend