Most physics options are not editable in placed instances of a Static Mesh with an Actor Parent.
1. Create a Blueprint
2. Add Static Mesh Component, Assign Mesh
3. Make mesh the root component
4. Place blueprint in level, select mesh component
5. Observe that most physics values are uneditable
to contrast - Creating a Blueprint that is a child of StaticMeshActor will allow values to be changed.
There's no existing public thread on this issue, so head over toAnswerHub just mention UE-57626 in the post.