I had to search for this to find out what it is. For the benefit of others, this syntax is from Extension:Labeled Section Transclusion.

Firstly, contrary to the description, a purge doesn't seem to be required; simply refreshing the page was enough for me to see it correctly. Secondly, this visual glitch only affects the user that made the change, and only affects them when viewing the page right after saving; any other user that goes to the page sees it correctly. Thirdly, this kind of syntax is incredibly rare in the article namespace of most wikis.

This is something we'd want to fix eventually, but given the above it doesn't seem to be pressing.