Formatting a card on the 50D saves all ML-related files, does the format, and then restores them--nice! But in recent versions the CROPMKS directory seems to be missed. This dates back at least to the 2016-12-29 version.
Reproduce by: create an ML card with an ML/CROPMKS directory and at least one crop marks file. Reformat the card in the camera. Power the camera off and on; note that ML complains about the missing directory. Mount the CF card in a reader to see that the CROPMKS directory has disappeared.