I have a sling on my Howa, I changed how I shoot it recently.
I found that if I had my sling under the sandbag at the forearm when zeroing and such, (the way I used to do it,) the POI was 2moa lower than when I shot it in field positions. I would put the strap under the sand bag because it would have more stability.
I concluded that the bag was holding the rifle down during recoil, but with no tension on the strap it was free to naturally elevate with recoil.
I'm wondering if the POI changes using the strap tight against your arm, like the strap being tight against the sandbag?
EDIT: I tested that idea again a few days ago, I concluded that my scope was screwing with me. My POI didn't change at all this time... I need a better scope.