Dirty encoder? Can you reassign that parameter, or assign another parameter to that encoder to test?
There are many reports on dirty/faulty encoders causing these sorts of issues. Here's one post I found:
https://forum.arturia.com/index.php?topic=94514.msg154736#msg154736But there are many others. If it's just an issue with that particular encoder (make sure you turn and slide others to ensure it's not another encoder messing up the control data stream), check if it's under warranty. If it's out of warranty and you've isolated the issue to just that encoder, try the following steps:
1. If you're lucky it's 'moveable' dust, remove the knob and spray compressed air at the base of the encoder while wiggling and turning to dislodge dust on the encoder/ring.
2. If that doesn't work, buy a no-residue electronic contact cleaner and spray that in the same fashion.
3. If that doesn't work, buy a lubricating cleaner and spray sparingly over the course of a couple of days to avoid soaking your boards (or worse, LCD) in oil.
4. If that doesn't work, then either replace the encoder or map around it and hope that the encoder doesn't start interfering with controls streams (keys not working, random notes firing, other jumpy sliders and knobs, etc.)
Hope this helps!
Edit: I should add the 'traditional' did you completely factory reset the KeyLab, but I usually assume that people have tried that step first