Hello can anyone advise me?

I've a Miele integrated fridge/freezer, problem originally was F4 code in freezer compartment
which turned out to be evaporator sensor which I replaced and cured the F4 problem but then I noticed food in the freezer was a little soft even though temperature on control panel showed -20. I then also replaced the temperature sensor inside the freezer compartment which made no difference and food was still softish in the freezer compartment even though temperature was showing -20.

I checked temperature of freezer compartment with independent thermometer which showed -8?

A list of things I've already checked:
1. Compressor working fine, cuts in and out and only warm to touch so no overheating
2. Fan in freezer spins freely and cuts in and out ok
3. Freezer defrost cycle working working OK (have checked by collecting water independently)
4. Super freeze button working on freezer OK, goes down to -26 on control panel but shows only -10 on independent thermometer

I think I had the temperature problem before the F4 but didn't notice it.
Running out of ideas so any help would be greatly appreciated.
Thanks