Yes its Scintilla, thats where the highlighting is, but I don't think its a bug, because `_"` is not a standard lisp syntax, see [here](http://www.gigamonkeys.com/book/numbers-characters-and-strings.html) so I doubt a report is worth it.
See [this](https://github.com/JeffBezanson/femtolisp/blob/master/README.md) on the subject of non-standard features in Lisps :grin: