Searching a bit, I found some videos and tutorials about the uf8 disassembly. I was a bit surprised that the screws are not exactly at the same positions as my keyboard, there must have been different chassis revisions. Would be nice to have a true service manual, especially if it already exists, though.
The good news is that disconnecting the pitch bend did the trick : the keyboard no longer sends random pitch bend events.
Best regards and thanks for your support,