Hi, I've just realised that my player has the same score of 58 on the MDC MDD and MDG items, but he's got a basic score of 78.

He probably doesn't have the right instructions.
Try to change his instructions to put him at his best NG.

Nims57: Il ne doit pas avoir sa bonne consigne.
Essaies de modifier sa consignes pour le mettre à sa meilleure NG

Thank you, I've changed the instructions and it's back to its original grade.

Yes, in the other posts this is the normal procedure.