Matte finish over satin finish?
A matte or eggshell, or pretty well any other finish, will cover over the old sheen. With it being freshly painted and primed, you shouldn't have any issues with the paint adhering. As long as the walls were prepped well before the coat you already put on, there shouldn't be any issues.

Keep in mind though, flat finishes are harder to clean, and in a bathroom with a shower, eggshell is is a good choice as a minimum. Many paints are mold resistant now, and there are several brands with eggshell being ok for bathrooms, but the flat sheens may not be.