Just finished updating and, man, what a HUGE improvement ! I've spend about 10 minutes trying my best to have one of the encoder skip a value, but I couldn't. They all work absolutely flawlessly now, FINALLY!
So I guess the culprit was probably that software acceleration feature that was maybe too sensitive, so that when an encoder was occasionally a little scratchy between two contacts, the software thought mistakenly that the encoder was turned rapidly, or something like that, and skipped a few values ?
Anyway, a big thank you ! It's like having a whole new instrument !