External Actors can hold stale references to Blueprint classes as editted instance properties

When a component is removed from a class that exists as an instance with external actors enabled, a deleted BP component on an actor will remain referenced by the instance.

Disabling External Actors or resetting the instance to defaults fixes the issue.

Create a BP Actor and a BP Actor Component.
Reparent the Actor Component to Decal Component.
Add the BP Actor Component to the BP Actor.
Place the BP Actor in a level with External Actors.
Save and restart the editor.
Edit the BP Actor and delete the component.
Reference view the level the actor was placed in and note that the instance of the actor references both the BP Actor class and the deleted BP Component class.

Packaging will bring the BP Component class into the package even though it is no longer used.