@eliot, here, i think it's not smae problem
but have you try with last SDL2.0.22 (https://www.morphos-storage.net/?id=1746996) ?
(if same problem)

And warning, now SDL2 support SDL_FULLSCREEN_DESKTOP:
https://github.com/timoinutilis/inga-sdl/blob/7b73df0ef16d1bb73868108574a15171fa353a78/sources/main.c#L99

If use this flags, window take fullscreen of Ambient and not real screen.