I was checking if T65696: Flow: display, behavior, and API failures when displaying a post revision still applies, and it partly does. I'm extracting this part out because there is leakage of an HTML attribute that could lead to an XSS.
The actual post is just 'C'. It's also a post, not a topic title. See https://www.mediawiki.org/w/index.php?title=Topic:Sjbg86fqz4m00em9&topic_showPostId=sjbg8hzaneecmqks#flow-post-sjbg8hzaneecmqks