I know about the limitation that you cannot use SkeletonGraphic(UI) with multiple textures, that's why I'm using SkeletonAnimation.
When I switch animations character is not properly displayed, but after a few seconds it is ok.
If I make bigger texture (so no multiple textures are created) everything is ok.
I though multiple textures is a problem for SkeletonGraphic, not for SkeletonAnimation.
I've latest version of the Spine (3.6.52) and unity runtime (spine-unity-3_6-2018-04-18b).
I'm using Unity 5.6.4.