"He who marches out of step hears another drum." ~ Ken Kesey
Definitely seems to be a chemical residue of some sort not getting sufficiently rinsed off.Do you de-haze every screen every time?Do you feel you need to de-haze every screen every time?Do you have this same problem if you don't de-haze?
After degreasing and your screens are drying how are they sitting in the rack? We used to have that issue due to excess degreaser on the frame puddling and running down into the image area. We started to rack our screens slightly angled which helped that problem as well..... But your just not getting all the chemistry off the frame and its running back into the open mesh.