If the buoyancy gets you deeper or higher should depend on how deep you actually are. Higher depths mean higher water pressure from above which would drive you deeper. Holding depth means that the depth rudders are set to give you some uplift against that. It's similiar to a plane, which too needs forward motion to make use of its wings
What I don't understand quite well is why you get bounce upwards when e.g. at periscope depth while standing still. Shouldn't the crew be able to let some more water into the flooding tanks to make the sub a bit more heavier ?