I had that issue with the K31/43 when I first upgraded it(on PS4) other day.
What I did was fully upgrade it, reset my upgrades, then put them back to exactly what I had it at, and the 6x worked proper after that.
I am trying to think if I did anything else.
I remember searching online to see if the scope was ever "fixed" originally....oh, there was something posted about it mattered about which perk is selected first, the left or right side.
So I picked the opposite, something like RLLL, went into Combined Arms, and it didn't work.
Reset it again, put it to what I wanted, I believe the LLLL, went back into Combined Arms, and it worked that time?
Or I might of had to try it a couple more times?(I honestly can't remember)
I do know the first time/couple times, it didn't work, but then it finally did, and kept working proper after that.
Not sure if it is still working though, as I only used the weapon enough to fully Gold it out, like 3 hours/150 kills, have not used it since.
~edit
Just checked, still at the proper 6x.
