Daryl,
It must be the inefficient nature of black powder that allows heavier charges to push the groups apart, since this is the opposite of what smokeless powder does. It makes sense though, if the recoil increases at a faster rate than the velocity, that would drive the barrels farther apart when fired.
I have noticed that in my 450/400 double, different powder charges give different vertical spreads. Starting loads shoot 4” to 5” apart, with the left barrel hitting 2” to 3” higher than the right. With maximum loads, the groups are 1 ½” apart and the left barrel is only 1” higher than the right barrel. If I could increase the powder charge even more, I wouldn’t be surprised if the groups crossed and the left barrel hit lower than the right, just like your Kodiak.

Greg