Amarok the problem was on my side. I have pushed version 1.7.3 of the Wordpress extension that fixes it.
It was a nightmare to troubleshoot because there were no errors, but I figured it out in the end. The problem was server-side, because I extended PostSerializer but not BasicPostSerializer, and when firstPost relationship is included in ShowDiscussionController, both serializers run before merging the attributes together, which resulted in the data being omitted because the data types for content weren't compatible.
