Unfortunately I could not reproduce your problem.
Could you please send us your exported assets as a zip package to contact@esotericsoftware.com?
We have noticed some bad playback behaviour in the _SkeletonData
animation preview window though (performance wise, playback being at 1fps). Also when the generated Material asset is selected, Unity seems to auto-update all the time without reason - which makes no sense at the Material.
Anyway, there is a certain chance that this issue will be gone when Unity 2019.3 goes out of alpha state.