Does anyone here remember a thread about density altitude that ran a while back?
I don't know what the outcome of this research was but it may be relevant to the issue, but I doubt it.
In my experience a vertical shift is related to hold/balance, have you ever had a day at the range when your zeroing and all of a sudden your zero is out, I always thought it was the rifle, but these days I am convinced the problem tends to be me changing my hold!
I built a HW77FT, it started life as an early 80's long 77, the barrel was shortened to 14 inches and with a stainless steel air stripper/muzzle weight, I discarded the spring/ball underlever setup and made a push button retainer that you have to push to open and close, that was it is solidly locked when up, not dependant on a spring to hold it.
internally I used a lightweight 210g 25mm piston running on bearings in a 26mm compression tube, the top hat and spring guide have 2mm clearance when the rifles cocked and the spring sits on a rotary thrust bearing on the guide to prevent torque and twist on firing,.
transfer port is 3.6mm standard length and a V-Mach spring and seal is used, it's set at 805fps max with JSB7.9 and I have never seen a problem with hold sensitivity ever since I spent a day messing about with different weights on the stock and barrel to balance it!
The stock is mk1 77 modified with adjustable comb, butt hook, and hamster, and an extended pistol grip and palm shelf to align my trigger finger with the Rowan extra setback blade.
I was quite surprised how much difference to poi was made by adding weight one end or the other, with a Bushnell 8-32 on it I ended up with half a kilo of lead in the butt to balance it so it sits neutral on the hamster where my hand rests.
once balanced to suit the rifle I have never seen a poi shift with it since, zero holds weeks later without need to reset.