Yep, usta get red stuff out of the bore of my Getz .54 flinter for years. And only my Getz, no other barrel. I've heard the "perfect load" comment, but discounted it because I got red w/ every load combo. Use only GOEX.
Noticed when I moved to Wyoming, the red gradually quit. Don't know if it's the lack of humiditiy or it just finally became clean. Haven't shot it much here in Idaho, but it's pretty dry most of the time.
Incidentally, through the intercession of the National Shooting Sports Foundation, small arms powders were exempted from the taggant requirement, including black powder. They convinced legislators that due to the small amounts used in shooting, taggants could dangerously affect performance. So don't fear, the Feds aren't tracking your powder purchases. Otherwise ATF woulda visted me years ago.