It could just be the denim. I've had some that I bought new, washed and tumble dried, as I do to all my bulk patching, and used as I normally do. that would not perform in any of my guns, no matter how I lubed it, or what powder charge. The denim just looked like it blew up like yours. Maybe bad weaving, or whatever. It was supposed to be 100% cotton, and burned like cotton, with no sign of any synthetics in the fibers.
Try different denim or ticking, or canvas, and see what happens.
On the other hand, since the rifle is shooting accurately, and if it is consistent, maybe you shouldn't worry.
-Joe Stein