Alright, from my brief testing it would seem this does not reproduce under Wayland.
My recipe above appears to be quite sensitive to details: I have a test user with a clean desktop (with little more customization apart from the extension), and there the recipe works just as written. On the other hand, with my main user account it doesn’t. To clarify: while the steps listed above fail to reproduce the issue with my main account, my main account does manifest the issue too; just not with those exact steps.
My main account starts up the Signal desktop client, Nextcloud client and some other stuff upon login, but so far I haven’t found which of those (if any) causes this difference.
I was, however, able to reproduce the ”see-through hole” effect under both accounts: instead of maximizing Gnome terminal (in step 3), I tile it to cover the left half of my screen, then start up Chrome/Firefox, tile itover the terminal window (i.e. to also cover the left half of the screen), before turning the display off and on again. Firefox then also manifests the hole, as does Gnome terminal when brought back to foreground from below. (Chrome does not manifest the hole, but then again Chrome has always appeared oblivious to the extension here.)
I should mention that I’m leaving the display off for about 4-5 seconds before turning it back on again. I’m not sure if that makes any difference, I’m just doing to to be sure that the ”I’m off” signal has time to propagate back to the system.