I have fired Winchester AA and Remington plastic wads with both BP and Pyrodex, and they melted. The whole cup is plastic, not just the base, so the fact that base wasn't deformed doesn't mean plastic deposits were not being left behind. Further, they did not come out completely with a bore brush. Leaving the deposits opens a whole world of problems, especially for the ML.
The original formula of Hoppe's #9 included benzene. It was a miracle cleaner, when the first plastic wads appeared and were used for hot powders like cordite and a few other or black powder shotshells. An avid target shooter could clean the barrel or barrels with one pass of a tight, Hoppe's soaked rag..., but benzene causes cancer..., so it was reformulated. The plastic got better, and the powders got a bit cooler.
I was lucky in that an industrial chemist gave me a quart of the vintage stuff as a thank you for volunteering some hours at a local range... and the benzene really dissolves the plastic out right away. This was the only time I've had an application for the stuff, and was dang glad I had it!
So as Articap suggests, protect the base of the wad. The problem with a muzzleloader is that few folks can actually visually inspect the bore, and a brush with plastic bits, and a clean patch, I found didn't mean all the plastic was out.
LD