Thank you so much!! Disabling Multiplane Overlays appears to solve it – and also fixes the occasional black glitches in Chrome (which seemed to manifest primarily on YouTube while UBlock Origin was enabled).
The disappearing menus in Unreal first appeared when I had been running two instances of Unreal at the same time (4.27.2 and 5.5.4) and had been copying and pasting nodes between them. I could temporarily get some of the vanishing menus back by clicking “Restore Down” to un-maximize the Editor’s main window, but then one of the two versions of Unreal would go completely black. So far though, disabling Multiplane Overlays using the registry files from NVIDIA seems to have corrected the issue. 