"He who marches out of step hears another drum." ~ Ken Kesey
IMO it is caused by heat from the bulb coupled with whatever humidity is in the emulsion. The day I went LED the problem immediately went away with very few exceptions of a freshly coated 110
Still happened to me even after I built my new screen room which is a constant temp of 72-78 degrees and 35-40% humidity, even if the screens had dried for a month and the films had been left in the same room. Happens more on thicker stencils/lower mesh.SP1400 I can coat, dry for 20 minutes, and expose without issue. Longer exposure of course.