Sounds like the minky “fuzz” is getting pulled up to the top. I think if you change to a needlepunched cotton batting such as Warm and Natural, or add a thin layer of Quilters Dream cotton next to the backing, underneath the polyester batting, there would be enough resistance to prevent the issue. You could also try changing to a size 16 needle. The smaller the needle hole, the more resistance to pulling Foxx to the top.