When files are added to a Unity Spine project, textures and a Spine model are automatically created. Textures bleed. If you check the "Straight Alpha Texture" checkbox, then the problem seems to disappear. Is it possible to make it active on default boot?
Straight Alpha Texture Q
apparently made a mistake. When the checkbox is checked, shadows appear as if there were no premultiplied...
See the documentation section below on how to setup any defaults for spine-unity:
https://esotericsoftware.com/spine-unity#Spine-Preferences