Help - I've replaced parts and can't solve this issue.

Temp in fridge getting too cold and freezing food.
Fridge set point is 8 deg C. Manual temp gauge tracks temp dropping to freezing levels but fridge digital temp consistently reads 8 deg C.

Fridge doesn't always do this. Sometimes it does keep temp at or near the set point (based on manual temp gauge) for a day or so. Fridge's digital temp display consistently shows 8 deg C. If we leave the doors open for a bit, the fridge temp display will rise above 8 deg C.
I have confirmed the fridge is configured to display the temperature and not the set point.

Eventually it gets the temp down to freezing - we have to regularly check the manual temp gauge and turn off the cooling using the button on the control panel.
When we notice the manual temp gauge getting too low, the damper door is typically open and blowing cold air. But not always - sometimes it's closed.

I have replaced:
- main board wire harness (includes thermistor): part WPW10119821
- main system board: part WPW10116216
Neither made any difference.

The damper assembly seems to be working - service test 121 shows damper opening and closing smoothly. But once during the test I did hear the stepper motor clicking - but it seemed to sort itself out after a few seconds.

No evidence of frost or ice buildup anywhere. Condenser coils under the fridge are clean (as best as I can get).

What next?