I'm interested to see what the issue is. I can't see anything that would cause that trajectory. My 54 cal Renegade is a couple-three inches high at 50 and on at 100. This is with 435 gr maxiball and 100 gr loose pyrodex.

By chance, is the gun rested on the same object and in the same manner when you switch distances? For example are you resting on a hard object when shooting at 50, but resting on a soft object when shooting at 100?