Would appreciate the groups thoughts on tandem axle issue

My first thought is the dealer may have placed jacks incorrectly under the axle to jack it up and caused the axle to lose the bow. Or as you wrote the axle is installed upside down which is unlikely since the spring pads would have to be welded on the wrong side of the axle from the factory.

Second thought is a curb or rock strike which bent the axle

Third thought is the tire went bad. This is common with original equipment tires.

Good luck in solving this mystery.