You don't mention the the granulation size or the charge weight, but I can tell you that the smaller the grains (fg,ffg,and fffg etc) will effect the pressure and velocity, the new "synthetic" powders ( 777 ,Shockeys, Cleanshot, et-al) are designed to replicate VOLUMN for VOLUMN loads for regular Black powder and they are SIGNIFICANTLY hotter powders and even at V/V they may be causing your rifle to shoot the same weight bullet faster, and in turn FLATTER. making the parabolic straighter and in turn shoot lower at shorter ranges.
I would try backing my load down a tick or two and trying it. I am also unsure of the effect of the lower ignition primers, but there will be an effect of some sort on burntime and accuracy.
If this proves in effective, I would go back to the Maxi Ball. It is designed to engrave the rifling when seated and is a very accurate projectile. The rifling grabs the sabot's too. but in my experience, the Maxi while a little stiff to load is hard to beat!
JMH2¢
DG