After RESTBase (patch will be linked after it is up) has deployed their editPreview patch, update PCS's mRequestUtil.defaultOutputModeForPOSTResponse (in mobile-request-util.js to MobileHTML.OutputMode.contentAndReferences.
Also need to update the description and headers in spec/transform/html-to-mobile-html.yaml.
This is blocked by a RESTBase patch, which is in turn blocked by this PCS patch[1] being merged and deployed.
[1]https://gerrit.wikimedia.org/r/c/mediawiki/services/mobileapps/+/596284