If you are sure that it is unburned powder then I would look to my lube. Too much or the wrong type can saturate the powder where it contacts the patch to the extent it doesn't ignite. Your process to lube the patch needs looking at.
The easiest way for me to check a patch for muzzle cutting is to seat a patch and ball using a large enough piece of cloth to get a hold of, then by holding the cloth on both sides you can pop the patched ball out of the muzzle. Then you can examine the patch for cuts.
Both Taylor (on smoothing the crown) and Darkhorse make good points. The one about using a strip of cloth, punching the ball into the muzzle then extracting it will show if seating the ball into the bore is what is cutting it.
I find the statement that only the 14.5 thou. patch cuts, while the .018" patch does not, - interesting in a contradictory way.(tighter patch not cutting is the contradiction to what is normal)
From that, I would assume actually firing the load is what is cutting the patch, however when such a thin patch is use, burnout is normally the problem- thus contradiction again.
If you are using a thick grease for lube, that might explain how there could be powder stuck to the patch. Something like lip balm would do that. Some people add Camfor oil & sell it as "Bore
Butter". I expect a thick coat of that might do what is happening.
There are much better lubes for hunting lubes - like Neetsfoot oil, or Track's Mink Oil which is a grease. I have never seen ANY fouling nor heard of actual powder granuals stuck to the base of a
fired patch, prior to this thread.
Something is dreadfully wrong with the loading and it is not the amount of powder.
For simple target shooting, many guns give their best accuracy with a water based lube. This cannot be used for hunting as a wet patch in your bore will start red-cancer.