Did you add o-ring recently? I got this issue with a few keys when I add o-rings to thick pbt. I had to push the o-rings in more deep to get rid of it.
I added o-rings with 0.2mm thickness to both my black and cherry keyswitches, but because the keycaps on the noppoo are quite thin, it doesn't work too well on the noppoo and it makes the keycap not sit on the switch correctly, so it's a bit of a problem sadly :/.
I could essentially take all of the keycaps off the keycool, apply o-rings and put them onto the noppoo, but it's just hassle at this point. I find myself switching back and forth in terms of keyboards, as I've ordered a new keycool 84 /w reds that should be arriving within 2 weeks.