Assigning a glossary to a parent does not change the glossary assignment for existing documents, even if they are linked.
Tip: When creating a new outline that will use a single glossary document, it is a good idea to create the glossary document first and assign it to the top-level document. Then all documents you create and link to the top level will inherit the glossary property. The initial glossary can be an empty document. As you create content, you can add terms to the glossary.