Hey! Check THIS out!
I seem to have found the solution:
What I did was I took it APAAAAAAART, (really didn't like to desolder the two wires connecting the main board to the protection board), and noticed the backlight module was spring-connected to the board. Maybe one of the springs was a bit bent, or maybe the module itself wasn't properly seated, I don't know. I put a dab of contact cleaner on the pads, tightened the self tapping screws in the top part fairly hard, and this seemed to have fixed it!
Here's the ultra poor quality video, I was holding my phone with one hand and manipulating with the other: (please also excuse my italian reinterpretation of the english language
)
https://youtu.be/SNntyelmeLAWell, I'll tag this topic solved (is it a right thing to do?)
Let's hope this problem never presents itself again.
Weird anyway, in fact I searched quite a bit and didn't found anyone having the same issue.