No volume appears when adding a blueprint of a custom volume class to the viewport. Adding an instance of the custom class directly does show the box, but additional functionality cannot be added via the blueprint.
Result:
MyVolume shows the wireframe volume box when added but MyVolBP does not
Expected:
Volume Box of custom volume class blueprints appear when added to viewport
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-47978 in the post.