I agree that it pretty much has to be vibration. The odd thing is that the rifle is a Weatherby MarkV and as such has a free floated barrel. It seems to me that it is difficult to get that to vibrate only vertically
Something is happening, but specifically what? When we talk about harmonics, we usually focus on the barrel, because that is where changes usually have the most obvious effect, but vibration includes everything about the rifle as well.
It could be that the entire barreled action is "flexing" in the stock enough to affect the radical point of impact shift. Since you mention another Mark V having a similar wide shift with different speeds of the same bullet, it might be something resulting from the way Weatherby's are built.
Several inches of impact shift are within the normal range at 100yds, with widely different loads, but shifts of over a foot? Now I am wondering if this is something Weatherby's do, or do in heavy calibers?
Here's something else to try (and keeping with only changing one thing at a time), try the same bullets with a different powder. Right now, all I see in common is Weatherby Mk Vs, RL-22 and a huge point of impact shift. Change the powder, and shoot. Similar results means we can eliminate that one factor, at least.
Plus, its the easiest thing to change.
Anyone out there with a different rifle in these calibers? (yeah, I know, not likely)