I've been shooting the .45-70 Marlin 1895 for over 25 years.
When I first got it, I was advised to use IMR3031 for heavy loads. I did so and accuracy was remarkable.
I never considered anything else. But I learned a few things along the way.
First, with heavy loads in which a lot of powder is used it helps to use Magnum Rifle primers. Not mandatory, mind you, but helpful.
But the most important factor when heavy loads are used is bullet crimp. Without a heavy crimp, the bullet starts forward before the powder is properly ignited.
I learned long ago that if I found a great deal of IMR3031 powder in my bore, just as you did, the bullet crimp was the number one suspect. And each time, a heavy crimp combined with a Magnum primer cured it.
I suggest you examine these two factors.