Glad you have sorted it, but:

"As soon as I open the service door in the corridor, and leave the room's door open, it works amazing."

That is the odd thing.

When you crack the door open is there a flow of air out of the room through the crack, or air entering the room through the crack?

Sounds like some of the air enters the room via the grille and the rest from the corridor,
OR some of the extracted air gets back via the grille and the rest via the corridor.

Might be worth a close look in the bulkhead to see what is going on with the airflow.

Has it always had this problem since new or is it a new installation?
Filter clean?