Has anyone else noted this behaviour in a Ruger #1 45-70?

Regardless of velocity light loads hit the target higher than heavy loads. At 100 yards a 336 grain bullet at 1400fps will hit the target 10 inches higher than a 400 at 1800fps from a bench rest.
I think that it must have something to do with the greater recoil of the heavier load.

Any other thoughts?

I've not seen that one Leo. I've seen #1's print higher velocity for a given bullet at a lower point on the target, but not what you describe.

The 45-70 is very sensitive to bullet weight. I have three of the things and every one of them will print to radically different points of impact with different weight bullets.
I think that may be more of what your rifle is doing, rather that velocity.
Paul B.

Well, I HAVE noticed that htere is a big difference in the point of impact of heavy vs light BULLETS in the No. 1 .45/70. Much worse than the .375 H&H Magnum version.

Yes, my No3 shoots low with 450 grain bullets (the heaviest I have) and even lower with heavy loads. I had (forgive me) cut the stock to install a recoil pad and thought I had the pitch wrong but changing that did little to change things. Seating the bullet out far (sized to bore dia. and paper patched on the back 3 grooves) helped a little but didn't have a chrony to see how much the velocity changed when working loads back up. I still want to figure that one out so I can shoot those heaavy loads, can't adjust sights to POI.

It sounds like barrel harmonics are causing barrel whip and the heavier bullet is exiting at a different point in the rise and fall of the whip. I have seen quite a few different makes of guns in different cartridge chamberings do this. It is why one must sight in different loads as there is no way to determine what different loads will do. Some guns will print near the same place vertically but move point of impact horizontally by several inches at 100 yds. Others as a Tikka I own put vastly different loads almost to the same point.