Hey! This may be helpful: Premultiplied Alpha Guide
Basically:
Your texture settings needs to match your shader.
Spine's shaders do premultiply alpha blending by default. So your textures need premultiply alpha checked.
If you use straight alpha blending, you need to use straight-alpha shaders like Sprites/Default and leave premultiply alpha unchecked.
You can check the recommended export settings here: Spine-Unity Export Settings
But as long as you know how your shader works, you can choose any settings that matches appropriately.