I hope you're on track to a solution, because your thread prompted me to reflect on and inspect my set up as I'd began getting vertical stringing with my .22lr shooting 100/200yd. This reminded me that in my hurried nature I'd added additional sand to my front bag a few months ago.
Today I inspected my rest and found that the post top surface where it mates to the bag base was loose even when torqued and allowed a slight amount of fore and aft rocking movement. Flat filing the post top and retorquing the fastener resolved the fitment issue.
Then I inspected the front bag and removed some sand, refit it, and removed some more sand.
My guess is the accuracy erosion I created will be restored and then some.
Today I inspected my rest and found that the post top surface where it mates to the bag base was loose even when torqued and allowed a slight amount of fore and aft rocking movement. Flat filing the post top and retorquing the fastener resolved the fitment issue.

Then I inspected the front bag and removed some sand, refit it, and removed some more sand.


My guess is the accuracy erosion I created will be restored and then some.
