The real problem is that the steel plate is not really flat. If it lies on the ground, the weight pushes the mirror flat on the wall but showed some delay, which is a shame, because the look is really great then, if the edge is resting firmly on the wall.
In our case it was not so bad because we backlighting the mirror with LEDs. In this case, the mirror is mounted on a frame with a distance from the wall. With embedded within neodymium magnets being pulled clean flat. Passts now.
Bottom line: See heading, and also bear in mind, of course, that it is quite cheap (if you researched well)