Mike,
Shooters have been looking for "WHY" for as long as we have been shooting and there are NO easy answers.
Perhaps it's what hasn’t been mentioned – “barrel harmonics”.
Each rifle/barrel has a favorite load and/or sweet spot.
Some, (a few) are also more forgiving than others and shoot different loads with minor variations accurately.
Accuracy itself being a relative term ...
You changed the recipe and now the projectile is leaving the muzzle at a different point in the node(s).
Therefore, the the point of impact will change in relation to the point of aim as will the size of the group(s).
Without altering the powder charge, patch material, type and amount of lube ONE item at a time to compensate, you just wind up chasing your tail.
As it is, you have a rifle and load that appears to shoot well.
Not knowing what discipline you are shooting, the question then becomes, is it a winning rifle?
If so, buy as many boxes of swaged round balls with the same lot number as you can afford.
Do the same with the lot of powder, lube and the bolt of patching material and anything else you’re using.
Then, shoot it until something changes …
Best of luck!
Cheers,
Smoketown
PS
Back in the day, I had a .222 Remington that was a ‘one hole rifle’ until they (Herters) discontinued its favorite bullet.
(I got rid of it when I used up the last of my ‘stash’.)