Full details are helpfully here:
This reminds me of the bug I wrongly filed against Flowless ( https://github.com/TomasMikula/Flowless/issues/37 ) before fixing it in commit de7e0643bf5471c4dbbda1798925a5f26e21fe48 which may be worth looking at as a comparison. In that case we had a listener which was modifying something it shouldn't, and was thus tripping up Flowless/RichTextFX. It may be the same here. Can't reproduce yet, but will try on Linux.