I have exactly the same problem. I don’t have a data logger, but looking at the thermometer from time to time I see -25c, but more seriously I also see -5c and the food is starting to de-frost before re-freezing. Obviously it is dangerous to eat this food.
I replaced the thermistor in the insulation at the back of the fridge 2 years ago. This is easier than it sounds and only took a few minutes. The fridge went back to working perfectly instead of freezing the milk, and it still works perfectly.
The compressor must be in very good working order to achieve -25c, so it’s annoying that I might have to throw the freezer away for what is surely a simple component replacement. But what is it ?
Any help greatly appreciated !