When dragging one set of bones/slots/attachments from one skeleton to another skeleton inside a Spine file, the draw order gets mixed up.
It doesn't seem to get mixed up in a way that makes sense?
It's not alphabetical as far as I can tell.
Any insight as to if this is a bug or if I'm missing something is welcome! 
ArtizensAnimator Sorry for the inconvenience. There is a known bug where the draw order changes when bones are moved between skeletons. You can find the issue ticket for this bug here:
EsotericSoftware/spine-editor806
As shown in the replication steps described in the issue, the slots under the child bones of the selected bone appear to be up after the bone is moved. If all the slots in the reproduction project attached to the ticket are direct children of the bone1 bone, and then the bones are moved between skeletons, the problem of the drawing order changing does not occur. This may be a workaround for you.
Misaki Thankyou so much for your swift reply!
Aha! One of our oldest and most distinguished animators! Glad you are still kickin'. 
Dragging things across skeletons is surprisingly complex. On top of that, many users don't do it at all. Those two things combined means it likely has more bugs than any other area of Spine. Please keep your eye out for any weird, especially crashes, keeping in mind what items you dragged across skeletons. We have only a few rare mystery crashes that we can't reproduce and it would be amazing to squash them. I'm pretty sure they have to do with dragging across skeletons.
HAHA! @Nate
It is my honour and pleasure to break spine all these years!
Only thing I've noticed repeatedly is the Spine interface just freezing.
I can still ctrl+s and close and reopen.
But it doesn't seem to have anything to do with skeleton draggin'
I shall keep my eyes peeled!!
That sounds terrible! I have never seen that. Any insight into what triggers it could be helpful.
You might try pressing each alt, ctrl, and shift key on your keyboard to ensure none are stuck in the down state, whether physically or just that the software thinks they are pressed. The key up event can clear the stuck state, if that shot in the dark is the issue.
HMMM it's tricky.
The stuck key theory doesn't seem right. Tried pressing them in turn, no success.
BUT!
Interestingly the mouse over the "SPINE" and "ANIMATE" still changes the mouse pointer.
ALSO the window resizing.
It's as if you can resize the window, and it still shifts the point at which you can click and drag the edge of the windows in spine...
but nothing visually changes.
I can't seem to reproduce the bug to happen on cue.
It seems to happen randomly.
I will continue to keep an eye on it!
Wow, that's really bizarre. It seems like the UI is working as normal, but rendering has frozen for some reason. That shouldn't normally be possible. You could try updating your graphics drivers, in case something has gone horribly wrong in the drivers.
