Nested Markup and the "Three Hops" Heuristic

This support article explains a possible reason why you might only be seeing an @id for a property where you would expect to see a whole data item. You might be seeing this when you select "View JSON-LD" in the Schema App Editor. You might also be seeing this on live URLS you test with the Rich Results Test or in your GSC Enhancement Report.

Schema App's tools create schema markup in a JSON-LD format. The JSON-LD on your website is deployed to individual pages but it also exists in a graph where entities (e.g Person, Article, Product) are connected by explicitly defined relationships (e.g parent, datePublished, price).  

When you author Editor markup, each data item you create will have a URI which serves as:

There are practical deployment and SEO benefits to using URIs as unique labels, and deployment instructions. The third use case suggests that URIs can also provide a strategy for mapping and organizing information about your Organization, its areas of expertise, and contact information.

This is expected behaviour, based on a rule of thumb we call the "Three Hops Rule".  It happens in our JSON-LD preview tool and in Production with some slight differences.

When a data item is nested down 3 layers from a top level entity and belongs to a different URL we do not include it in the markup preview. In this case, if the data item is 3 "hops" away from the primary entity, we will specify the relationship or property, and the @id or URI of that but not its contents of the data item. This is done to lower complexity when reviewing markup. You can always navigate to the data item itself in the Editor to review it.  

When a data item is nested 3 layers from the top level entity and belongs to a different URL, we do not include it in the markup. We do this because it is likely that the content referenced in that nested data item isn't actually present on the page in question. To avoid adding markup that doesn't actually reflect the content on the live page, the software abbreviates or truncates the data items that it deems to be "too far away" to show only the URI or URL of that information. If the content being described exists on that page, consider making a data item that "lives on" or has a URI that reflects that URL of the page.