My understanding is that one may not descend below the MDA , without visual reference ; therefore the habit of adding 50 feet for a Cat C aircraft will hopefully ensure that in the event of a Go Around , initiated at the 50 foot point , the a/c will not go below MDA ( and you will not fail your check ).
With a DH , the missed approach must be INITIATED at or before the DH and a subsequent sag below DH is acceptable.