In my experience GPRs, especially when new, tend to have very sharp crowns. Short term I've learned to push on the short starter, slowly adding pressure until the ball passes into the bore, rather than "slapping" the short starter. At least when my guns were new, if I slapped the short starter it would result in a cut or tear of the patch almost every time. You can check on yours simply by seating a ball, then blowing it out with a CO2 discharger and checking the patch.
The sharp edge will eventually smooth a little and the condition will go away. If you get impatient like me, press a piece of 320 wet/dry paper down on the muzzle with the ball of your thumb and rotate it a few times with even pressure.
Dunno how many shots you have through your rifle and it's recent history, so it could be sumpin else. If I understand you right this is a new condition, so something has changes, whether in your loading technique or in the bore. If you decide to try a thicker patch, try the seating method I described, because if anything, it will have more of a tendency to cut on a sharp crown if you simply slap the starter.
