I'm back with an update and with an hypothesis of why the problem may have originated. First of all, excuse me if my english writing isn't on-point since it's not my native language.
I tried to unscrew the four screws on the main frame (photo link further ahead), and noticed that only one of them was "unscrewable", while the other three were EXTREMELY tight. Blue square represents the unscrewable screw (...), red square represents the really tight screws.
On a second note, I noticed that the spring cover (what goes directly screwed over the string) has shifted slightly (photo with arrows pointing further ahead), and this might have caused those screws to be so tight. My theory is that improper force balancing of the spring caused the whole spring cover to shift to a side, leading to some deformations that might cause the squeaking sound
Also, after applying more WD-40, squeaking won't go away. I couldn't get a lithium based lubricant since COVID measures in my country are kinda strict.
Any thoughts/ideas are welcomed :)
PD: I haven't reached support yet. Will do this month.
Main frame with colored screws:
https://imgur.com/JA5fKll
Main frame with arrows pointing spring cover gap:
https://imgur.com/a/Lz7fDDd