Yes, further research convinced me that it is the consequence of the ill-advised animated wallpaper feature Google "invented" in Android 11, and which no one seems to be able to turn off. This is confirmed by the fact that if I use the same "native screen resolution" image file for the lock screen, the photograph is displayed flawlessly there, but is mangled on the Home screen as in the example I posted above.
It would be nice if GrapheneOS could dispense with such useless fluff, but I understand why the developers have more pressing tasks to attend to...