When a collection containing works is moved under a parent collection, the works don't go with it

Parent collections should contain the works that are in all their respective child collections. But! sometimes they don’t, when the collection was added to the parent collection after already containing works.

Steps to reproduce:

1. Create a collection, Collection A.

2. Add some works to Collection A.

3. Create a new collection, Collection B.

4. Edit Collection A and make Collection B a parent of it.

What should happen:

Works in Collection A should also appear under Collection B once Collection B is a parent of Collection A.

What actually happens:

They don’t.