The act of temperature control is a mixing process. The defrost position is maximum heat and dehumidified air via the A/C being also on.
When in EC the system mixes heat with ambient and you get heat. When in any other position the A/C is on so there is not the distinct heat position unless you are very far from the set temp or "Max" temp setting. If you are not getting the proper mixture eventually then there is a problem.
Bosch Master, ASE Master, L1
33 years MB technician