Took me a while to figure this one out. Our job to ensure data was consistent wasn’t taking into account the situation where a value would be set to nothing (like a deletion). This would only happen on short topics like the one you found. Thanks for letting me know:
https://github.com/discourse/discourse/commit/94a4af6af77b3d6366f4c7c685d9edd2b7e9be43