This bug is a bit complicated to explain, so i'll explain what I did to see it step by step.
- Create a Spine object that has a mesh as an attatchment to a Slot
- In setup mode, set this slot so it doesn't have that attatchment selected (the slot will show no image, just as it had alpha 0)
- In animation mode, make an animation where you deform that mesh drastically and at the same time you set that slot attatchment to the mesh. This way, the slot will show the mesh and you will see just fine the deformation in spine.
- Load that skeleton in a LibGdx project. You will notice that, no matter how slow you play the animation, it will show the mesh undeformed exactly for 1 frame before doing the animation. If you play the animation slow you will see just one frame, and so you will if you play the animation fast.
I would fix this setting the slot to alpha 0 and turning it to 100 at the moment the deformation starts playing, the problem is I have to modify the setup color of this slot at runtime, so I can't use color keys for that slot.
I Believe this is a bug since it doesn't cause that weird artifact to show in spine, and it does on the libgdx project.
PS: I used last libgdx runtime