Have exactly the same issue, 2017 S3.
Local (trusted) garage flushed the heater system through. Only obvious thing on VCDS(?) was that the internal thermostat in the passenger side for the clima control was reading 10c higher then the drivers side does.
Get moderate heat when fans are low speed (1 or 2) but this disappears when I turn the fans up.
Radiator gets hot, inlet/outlet for matrix is fine.
possibly narrowing this down to the control unit in the dash itself. What's involved in replacing this? Any coding etc?
Best
