It was a file naming problem.
You had both a .atlas file (new version) and a .atlas.txt (old version). The runtime was using the old atlas with the new png so it had the wrong coordinates for the regions.
The Spine-Unity runtime seemed to fail to detect that you had two atlas files. (one extended .atlas.txt, and the other extended .atlas).
We can assume the first time you exported the atlas that Unity detected the .atlas file and automatically renamed it .atlas.txt. But after that, I think it just broke, as the case where it detects both ".atlas" and ".atlas.txt" hasn't been handled yet. Or has it? I haven't actually gone through the importer code myself.
USER SOLUTION:
Just delete all previous exports. Export a new set of files, and in Spine's Create Atlas/Texture Packer settings, change the atlas extension from ".atlas" to ".atlas.txt".
You shouldn't be getting this problem any more after that.
Note to the animator and the programmer:
Spine exports 3 files. A png, a json, and a .atlas (.atlas.txt for compatibility).
Spine-Unity's importer generates 3 more files (though not 100% true, think of them as having a 1:1 correspondence with the original Spine exports): a Material for the image. an AtlasAsset for the atlas, and a SkeletonDataAsset for the json.
That makes a total of 6 files with any regular set of Spine assets. Any more and you should be suspicious if you didn't have some kind of special setup. I know it just turns into a jumble of files most of the time. But it's handy when you need to figure out what broke.
Last tip:
You should trim/crop your Shadow.png image to remove the empty transparent areas.
That's extra work the GPU doesn't need to do, and wasted space in your atlas.
