Update: Turns out this is caused by viewing a large image and then zooming in. At a certain zoom level the app goes into fullscreen, hiding the navigation buttons and top bar. It then never exits this mode until you quit the app.
v0.9.0-3 Pixel 7 Pro Android 16
[I edited my screenshot to add the border and the word “SCREENSHOT” because the full size preview was making it very confusing to view the post in the app. 😅]
Oh, that’s it! That must have been what I did, and now I can reproduce that every time. I guess I should make a gitgub bug report.