I expect this was another 100yard target, stretchman. I see nothing wrong with the target and obviously from that
there is nothing really wrong with the patches, which appear to have done their job until the muzzle blast destroyed the
edges. You can see most of it is just unwoven, not cut or burnt. Fraying happens to the edges, especially with such heavy loads.
A 10oz or 11oz. denim would likely have have fared somewhat better. If you are happy with the accuracy, then so be it.
I might spend the time trying denim in 10 up to 12oz. just to see how it does, in comparison.
If the linen is loading well, shot to shot, then that's the important thing.
I see you used bore butter. Many folks have found that stuff wanting and have switched to Neetsfoot Oil or Track's Mink Oil
as a better lube, especially for hunting in cooler weather.