While I was working on Spine animation in Timelines I noticed that the editor preview was broken.
The skeleton behave like you try to play an animation on a higher track index but there are already animated keys on a lower track.
I figured out that this was caused by the base animation of the SkeletonAnimation component. Everything works fine if the "Animation name" is set to None in the editor but once you set an animation on that field, it produces what I described above.
I searched for a bit to find where was the issue but couldn't find out. Is there something I can do to fix that ?