Heating issues HELP

[Philgb4]

Hi all,

I've been the proud owner of my 2016 b8 Passat for a number of years and never skipped a beat. However the last few months have started to take its toll.
After a recent service, it was identified that the silica bag in the coolant reservoir split and blocked the heater matrix.
I was experiencing no heat output at all. A common fault. But was also difficult to diagnose as so many other things don't appear right.

I've since had the heater matrix changed and there is now improvement. It heats up and outputs 'some' heat.

But the heat only seems to be produced on one side at a time. This can also change from passenger side - drivers side. But at this time, the heat only seems to be produced at the passenger side. I checked the drivers side actuator motor which seems to operate all the way round with no problems. Any ideas? Could it be some sort of controls issue?