Unfortunately we could not reproduce any issue with the git project you provided. Each part GameObject with a SkeletonPartsRenderer is rendering in the order of the SkeletonPartsRenderer Order in Layer property, as described here as well:
https://zh.esotericsoftware.com/spine-unity#SkeletonRenderSeparator
Please note that the separator slots are separating your mesh in the draw order, not according to the bone hierarchy. Did you perhaps expect different separation?
In general, please always be sure to test with the latest version of the spine-unity runtime when encountering any issues.
如果插槽数量过多的情况下如何把某个SpriteRenderer插入到中间，有啥好办法吗？
You can insert a SpriteRenderer in the middle of a skeleton by setting the Order in Layer at each SkeletonPartsRenderer and at your middle SpriteRenderer accordingly.
