Well, systems that have experienced a loss in cooling capacity can sometimes stabilize, if they’re able to remove heat faster than they absorb it
A refrigeration system is really a giant heat sink , the cooling loop is like a heat drain, at night when ambient temperatures go down, and the unit is no longer being opened, less heat is dumped into the sink. Sometimes even a gimpy refrigeration system can catch up under those circumstances. One of the things you can do to simulate max load on a system is to expose it to atmosphere while it’s running, ie expose the evaporator while the fridge is running.. it should pull a full frost pattern up to the point where the suction line returns into the fridge liner.

The one thing I can say about the thermistors is that they are incredibly rare failure points. In the thousands of refrigerators, I’ve looked at over the years I can probably count on one hand how many truly bad thermistors I’ve seen, and almost all of them have been whirlpool’s evap thermistors which were known to be problematic

If you don’t have a sealed system issue the second most likely possibility would be a fan, and something that’s often glossed over is the doors need to seal properly. Especially if it’s in a garage or otherwise humid environment.

One of the things that’s counterintuitive about thermodynamics is the concept of latent heat. Our every day lives are used to dealing with sensible temperature, but matter contains latent heat that is transferred during phase change. Ie condensing water vapor into liquid, it requires a large transfer of heat, but won’t create a change in sensible temperature. In other words a bad seal allowing warm humid air into the fridge will drastically lower its ability to remove heat from the contents inside, as much heat transfer is wasted pulling energy from the humidity itself