Parsoid relies on the expandtemplates api call to get the expanded wikitext for transclusions and https://en.wikipedia.org/w/api.php?action=expandtemplates&title=Category:Engineering_disciplines&revid=746873858&text={{Commonscat|Engineering%20by%20type}}&prop=wikitext|categories|properties|modules|jsconfigvars&format=json returns empty output which is what Parsoid is displaying.

See T148118#2716884

To expand on T148118#2716884: Category listings are a widget rendered by the skin, and are not part of the actual page contents. The information is available through the action API, and can be rendered by the skin / client in a device / context appropriate manner. Some clients might go with simple lists, while others might prefer a category cloud.

Example query: https://en.wikipedia.org/w/api.php?action=query&prop=categories&titles=Albert%20Einstein

If the existing category tree render is preferred, then the null skin idea proposed in T114596 could potentially help to expose the default skin rendering. A small complication for category pages might be that they normally combine page content (category description) with a skin-provided widget (category listing). Ideally, the null skin would only expose the skin portion, and leave the content portion to the client.

I'm not sure what's your definition of "skin-provided widget", but categories are an integral part of page content, including on category pages themselves, and should be included just any other content even if generated dynamically.

Anyway, if Parsoid doesn't want to do this then the problem arises of an API to get the entire content of a (category) page, cf. T153980#2897321