Air-conditioner in heat mode
Air conditioners cool by running air through a cold coil (the evaporator). If the evaporator cools the air below the dew point, then water condenses, drying out the air. The excess heat is put outdoors using a hot coil (the condenser).

When you run a mini-split in reverse, it changes the evaporator into a condenser, which gives off the heat collected from the outside coil (now an evaporator). This heats the air, but it does nothing to the humidity, since the air never goes below the dew point.