Taxonomy search doesn't return nested results

We can get a specific Taxonomy search result if we have content directly inside the taxonomy term but it doesn't give a result if we select a parent/ancestor taxonomy node.

Is there a way of having the Taxonomy search return all nested results rather than just the direct results?

Surprised this hasn't been asked before so it may be we have set something up incorrectly. When we search for '-Any-' we get the correct results.

Eg.
Chicken
- Egg
-- Yoke
-- White
If we have content in 'Yoke' with the word 'yellow' and do a search in 'Chicken' I would still like the result to show because it is nested.