Could it be the fleece causing an issue? i.e. human detergents or softeners that are actually transferring to the gp's skin? I know over the years, I've run into issues with some clothes detergents where it causes irritation on my legs.
Also, I tried fleece for a couple weeks, and noticed all my sows were getting dirtier undersides to the point it was discoloring their underside fur, despite changing the fleece routinely vs. using Aspen wood chips where they are always clean. I switched back to Aspen and the discoloring stopped. My guess is the wood chip bedding actively "scrubs" their fur and wicks up moisture better than the synthetic fabric, and aids with cleaning.
IF they are laying on the fleece and peeing, and staying on the pee, their undersides/groin will stay wet until they move vs. wood chips which at least allow air pockets to be underneath + the dry wood absorbing residual. IF they have a decent amount of calcium in their urine, and that's sticking to them, that might cause an extra layer of irritation and might be the "skin peeling" you are seeing. If her urethra is inflamed, and her urine is spraying "back" towards her rear vs. straight down + calcium content + fleece sticking to the underside, it might be a vicious circle. Just guessing on my end.