Unobtainable, out of curiosity, did you notice this shift with the large aperture, small aperture or both, and what model/brand BUIS?
Wannabe, in the Troy BUIS at least, we notice windage shifts going from large to small aperture. This minute, I am forgetting which way the shift was. We noticed it when my wife was zeroing Troy BUIS on a AAC .300, and she shoots good enough groups to clearly see it at 50 yards. (I just use the small aperture.)
Likes pretty much everything in every caliber.
Rosh works sl1 up front and magpul MBUS rear. Small aperture only.
I have not tried this with my T2's yet, but with T1's and Comps, I have never seen this kind of shift. I usually zero my irons without my RDS on, and then shoot them through the RDS during training.