Zero your gun from the prone. If you have zero shift while on a bag, tripod, etc. It can only be one of two things first and foremost you the shooter and your ability to implement the fundamentals of shooting. The second is equipment problem either with stock/chassis ,torque settings on action screws, proper scope mounting, tripod etc.....check it all. A tip you could zero from the prone with a bag as well to verify equipment. You shouldn’t be able to apply force to the rifle in other positions that would cause your zero to shift. Once again fundamentals on marksmanship first. I have had one piece of gear that created weird problems for me and that was a Harris bipod, shooting off the bag from prone reenforced my rifle was fine.