Your problem is you do not understand the physics. Your bullets precess when fired. The rate of precession changes as the bullet settles down.
When the bullet leaves the bore the higher precession results in a faster deviation from the line of sight.
As the bullet settles down and becomes stable it deviates at a slower rate from the line of sight.
This slower deviation from the line of sight can result in groups at a longer range that are not as widely spread out as expected. This is NOT a case of a bullet veering back as you try to claim. Rather the bullet is now more stable and is deviating at a slower rate from the line of sight. Until you can comprehend that the rate of deviation from the line of sight can vary with the precession of the bullet you will never get why some of the rest of us are talking about.
Yes I have owned a rifle that does this since 1972.
As far as defining vintage shooting you are stretching the definition a bit.
Your black powder is no more vintage than any other modern manufactured powder. It is just an older recipe. It is no different than shooting your modern manufactured Sharps or Highwall replicas.
The twist in your bores does not match originals nor does the steel in your barrels or actions.
Since most of your components and equipment are recent manufacture I don't really see how you can call it vintage shooting.
You use modern boxer primed brass with non-corrosive primers right?