My 300 WinMag Blaser (not as powerful primary extraction) with a GEMTECH ONE works just fine and cases are spotless.
My best guess is that after the bullet exists and the case has shrunk down, there is still enough residual pressure in the can to send some crap back between the case and the chamber. Rather like you can get with low velocity cast bullet loads (although the cause is different.
How long since your can has been cleaned ? IMHO something is keeping the gas in the can after the chamber pressure has dropped.
Can has less than 75 rounds through it.