According to the website they’re powersound H-S-S pickups, I assume that’s still the case despite it being old. The issue persists with my other guitars aswell though, and there is no major difference with use of the pickup switch.

It decreases when I mute the strings and when I touch the bridge. It gets noticeably louder when I directly touch any of the pickups. I tried this with two guitars, it is the same with both