@plainklaughter Glad it helped!
Does this situation have something to do with the content of the animation?
Yes, the problem only appears when all enabled attachments have vertex color alpha of 0.0, which is only the case with all attachments having Additive
blend mode and Advanced - PMA Vertex Colors
enabled. Then Unity will treat them as "nothing is visible" and discards the mesh, even though the shader would later interpret alpha 0.0 differently.