I posted it yesterday to Bugzilla and they seem to be taking it seriously as they have upped the priority to highest. It concerns indexing, so I wondered if any of our indexers had noticed it.
Sometimes, when you set index entries which are different from the underlying text, you get unwanted italics or bolding, which can extend over several lines. You can remove them, but if you save and re-open the file, they come back. Both LO4 and LO3 are affected.
The bug number is 69386 if anyone is interested.