My 8Kx is from 2021 (ser2075). I had it in the official Pimax cover. I just took it out of the cover, and all of the blue surface is VERY sticky.
Water and soap of no value. I did find some Isopropanol Alcohol and some rubbing would fix it. But can also remove all the blue and leave Black.
Background:
I live in a very hot climate (Zhuhai, China). The room where the 8KX is, can get very hot over time. I put my Air-con on to 26C. So it is probably a long term hot room problem.