Just because the light is on doesn't mean the door has functioned properly. Or, in other words, if there is a vacuum leak the light won't reflect it.
Improperly functioning recirc doors are a common cause of poor duct temperatures. Most if not all models go to the first stage of recirc door closing upon choosing AC. If the door stays in the total vent position it will cause 10-15deg rise in duct temps on 126,124, and 201 bodies, maybe more.
Bosch Master, ASE Master, L1
33 years MB technician