Maybe that's why ingot molds are cast iron or aluminum instead of sheet metal?
I wasn't being a smart a$$ but lead sticks to sheet metal, or that's been my experience and now yours. Also, if you're casting many ingots, the sheet metal in the pans will get very hot very quick, as you probably learned, and it will take forever for the ingots to set up. If you're casting many ingots all molds will do this.
I've never know anyone to cast ingots in a muffin pan nor have I. Perhaps there is an answer to you question and I'm curious to learn it.
Lee makes some pretty good and not terribly expensive ingot molds. I have two and a couple RCBS. I prefer the RCBS.
Vic