If FD gets depressurised the door gets unlocked automatically.
This happens when there is a pressure differential between FD and cabin. We (or at least I) don't know what that differential is; we don't know how tight the air seal is between the two spaces. If airflow between the two spaces is not completely constricted, a slow leak in either space to the outside would not open the FD door on a pressure differential. Below about 10,000 feet, there probably wouldn't be enough of a differential to unlock the door either.