unexplained staining! Please help!

Sorry, posted this earlier but messed up the thread title.

Hello, I am in need of some help!

I work for an artist studio that has been producing machine embroidery on linen. The embroidered areas are quite dense with stitches. We notice that when they are done with the embroidery process, a halo-like stain appears around the embroidered area. It looks like it could be an oil substance, or possibly wax? We have no clue what is happening. We can clean the stain off, but it often comes back. It seems as if if the linen is leaching oils from the thread. Sorry, hard to explain.

With a little bit of research, I found that nylon thread has a lubricant coating. Could this explain the staining?

Has anyone experienced this? Does anyone have any ideas on how to avoid this in the future?

Thank you so much,

Brody