Have you tested the temp of the 'hot' water in the washer? I was surprised the hot washer water was only warm in my machine. The new machines that have water sensors do not fill long enough to get the water hot unless connected to instant hot water or directly from water heater. For me to get true hot water I have to fill and drain the washer and then start over. Wet fabric will shrink in a hot dryer so using hot water isn't necessary for shrinkage. I like Pellon premium muslin. I've never had a problem with it and some of my used quilts with the muslin are decades old.