yes, possible. My #1 was a soot thru umbrella (possible culprit?), #2 large Bron Boxlite, 3 and 4 were just gridded with barndoors.
There's the basis for a comparison that might be decisive. See what you get from the shoot-thru umbrella alone without the others. Then compare with just the Bron alone. Etc. I'm betting the Bron is not the problem, and for that price, it'd better not be!
The bad material not only fluoresces, it can also have a lousy color cast that just won't come clean. The fabric looks nice and white, til you hit it with a strobe.