01 BMW X5 temp. 1/4 when heater is active

[Boopunn]

Only when I turn the heater on does my temp gauge go down to quarter at idle .
When I am on the highway it is about 40%. Any other time of the year it is centered and perfect until I use the heater.

The complete cooling system was replaced 40k ago when rebuilt the 3.0 engine from bottom up. Radiator,hoses,sensors, thermostat and water pump. I also changed the heater core solenoids that open to let the water flow for the heater.

It had the older type thermostat that you can not remove the thermo heater and last year I replaced that one with the newer type that the heater will twist out of the Thermostat housing. Still no change.

I have backflushed the system to make sure there were no blockages in the heater core.

Could it possibly be the heater control board in the dash?

Does the temp signal go through the heater control board for the climate control before it gets to the dash?
Thanks for any advice.