In a Quark world-gen-modifed level I discovered large spheres of paper walls underground. They replaced both stone and air. An X-ray texture pack clearly showed the spheres. I saw them in a small patch at one place at the surface. They did not fill a dungeon I found.

Under ground I also found a few colored clay pots. There were only a few but this seemed out of place.

I am using the most recent forge and quark versions. Also running Twilight Forest. Minecraft 1.12.2

This is just a typical case of ID corruption that happens with mods, it's not quark's fault in particular.