I don't know if its related, but i was creating a new project, and i found that if you create an animation without bones, when you drop the prefab in the scene, Unity can't find the sprite you use in Spriter (the first time, if you save adding a bone in Spriter, go to Unity, then remove the bone, it shows properly)
i dont know why anyone will create an animation without bones, but i thought it will be a good idea let you know about that