This is a Unity bug and happens even without Spine.
We haven't heard from Unity yet about them having fixed it.
13 October 2017
Good news, everyone!
Just tested SkeletonGraphic (and non-Spine cases of the dynamic mesh leak) in 2017.2.0f3
It looks like they fixed the bug.
No update to the Spine-Unity runtime necessary.
(Mind also that I was trying out some new features in 2017.2. It looks like Unity editor's new windows and features might be a bit unstable, and I've also read some stuff about UI performance regressions. I might hold of and wait for 2017.2.1 if you're nearing a critical deadline.)