I have this exact issue going on with my Keylab 88 mkII but with my A key (2 notes left of middle C). I have had it for little over a year now and I just opened a ticket with their support and I will be sure to update you here when I get a for sure fix.
I tested on my regular studio PC and a macbook using ableton, garageband, and I opened Piano v2 without a DAW and the issue persists with all Software/Hardware changes. Firmware is all updated to the most recent and I was still having this issue before the firmware update (Just updated yesterday) and I am pretty sure its going to end up being a hardware defect but if you haven't already, I highly recommend opening a ticket so they are aware of the issue as well. They have been sending me suggestions like using your thumb to push the key down and back towards you to pop the key out and readjust it by pushing it back in but it didn't help in my case. (Maybe it will for you so its worth a shot)
Ill keep you updated. Just wanted to let you know that you're not alone! :')