I have found that with mutable-generated skeletal mesh, it will have strange deformations, such as vertex weights that have been incorrectly calculated [Mutable BUG 001]

Mutable currently does not automatically detect if a SKM has more than 8 bone influences per vertex. Probably what is happening here is that your SKM has 12 influences and those larger than 8 get remapped to other bones. You can manually change this setting in Engine\Plugins\Mutable\Config\DefaultM…