Delivery API does not return custom index values from IContentIndexHandler

We’re using the Delivery API in a headless setup, and all our content nodes have a theme field. The logic is that this field should inherit the value from a parent node if it is not explicitly set on the current item.

To support this, we populate the theme field during indexing via a custom implementation of IContentIndexHandler. This part works as expected — I can see the correct inherited value applied to the theme field in the Examine index.

However, when querying the content via the Delivery API (using Swagger), the theme field returns null (i.e., the raw content property), rather than the value from the index.

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Shouldn’t the Delivery API use the values from the Examine index (where I’ve updated the field), rather than returning the raw property from the content item?

Or is there a way to configure or extend the Delivery API to return values from the index?

I’m certain the query is made using the correct culture and content root.

Any insights or pointers would be appreciated — thanks!