Some update,
I spent yesterday's night repairing the faulty keypad, 2 keys were non responsive. The responsible was a damaged via on keypad's PCB, with a pale white stain around it. An enameled wire and 2 small welds later, it is back on track, good.
If it could help anyone, I enclosed below the keypad's pinout.
Now, back to the «lower scale» stability issue. If you take a closer look at previous' post picture, you can see a faulty column, right next to temperature bar. This appears after ~5minutes power-on so one can assume this might due to microbolometer warm-up?
Using «picture in picture» mode, thermal image sides are cropped-out, canceling the erratic behaviour.
Interesting...