As far as I know, an actor will not accept an animation blueprint that has a different core skeletal mesh than what it has.
Based on the unreal documentation; Different means extra or missing bones WITHIN the mesh itself, any extra appendages would be fine.
For example, two men have the same Sk.Mesh even if one of them has 3 arms, and the other only has 1. Then the core Skeleton would have 1 arm, and the other 2 are a welcome addition.
But if one of them has 2 spine bones and the other has 3, then it’s a different Skeleton entirely, and the blueprint won’t work.