Yes really not all that uncommon. It raised some of the same questions as yours in my mind. Separation seems to occur or start in area between disk and high hat hub of rotor. Where the metal is thinnest. Seems to take a chunk out of the hub of the rotor sometimes as well. Probably about time of total separation. Mercedes wants less than 5 thousand runout on a good disk so borrow a dial gauge and stand if you do not own one when putting on the new replacement rotor and check it. Also make sure your front wheel bearing is set up properly. Mercedes proceedure for that is very easy as well to do. With under torque front wheel bearing will probably contribute to actual rotor wobble or runout. Certainly can't help. Clean area of rotor hub contact well to help achieve under 4-5 thousand wobble as well. (did not look up spec but pretty certain) Imagine failures are partially stress related as that hub is pretty deep. On the incidents of mine found calipers to be in good shape. Others may vary I imagine.