When the transform is actuated for a static component only its pre animated state is saved, but its mobility will be set to movable. This causes its children to become movable as well. When the component's children are not animated themselves, there is no pre animated state to restore, so they remain movable. If they are animated, their pre animated state is saved after they have been made movable, so it will be restored to the incorrect value.
The cube's mobility will change back to static, but the sphere will remain movable. If you add a transform track for the sphere it's mobility will still remain movable after the sequence is closed too.
There's no existing public thread on this issue, so head over toAnswerHub just mention UE-92817 in the post.