Deleted post
a.d. files: https://0x72.itch.io/tilesetssplitter - hope it's useful
a.d. pivot - I'm pretty sure you can move pivot in Unity. Center of one frame might not be center of another - not sure how it's supposed to work, I guess adding padding or something. Feel free to alter the files so they suits your particular stack : )
You might find this article interesting: https://blogs.unity3d.com/2018/01/25/2d-tilemap-asset-workflow-from-image-to-level/