My custom shop Kelly had a lil dive until I bought a anthology gear wear strap. They're made from saddle grade leather, so they form to you're shoulder over time. That stopped the diving after a while.
Yup!..... All my Kelly's did except for my San Dimas era ones. I didn't notice it as much on my Explorer's except on my E2. I did a little but nothing as bad as the first runs of the Bernie Rico Jr. Vixens!! Wow. You couldn't keep those things anywhere close to a standing playing position.
I've never had to re locate a strap button. I simply picked up a nice 3" wide leather strap and it pretty much stopped all nose diving. Even with my Kelly's.