Parsoid doesn't handle all of the many corner cases found in non-mainspace or heavily transcluded pages. We should only use RESTBase for mainspace pages (except the main page) and fall back to MediaWiki API for everything else until T153980 is fixed
Unfortunately, this won't be an easy catchall fix as I'd hoped. The MediaWiki mobileview API doesn't give us the category listings, either. I guess it would take some more work in MobileFrontend.