This information is from a W126 publication, but the Automatic Climate Control (ACC) is at least similar, if not exactly the same for the W201.
In the three button positions that have double arrows, and when in Heating Mode, outside air will normally be emitted from the center vents, though occasionally warm air will come from those vents. The occasionally bit is a function of the cabin temperature, the programming of the ACC, and the condition of the vacuum actuators.
If the ACC is behaving correctly in all other respects, the vacuum actuators are the likely culprit, in that the blend door remains set to outside air and does not switch to heated air.