@davidfor : on the principle, you're probably right but i've yet to see an example in a paperbook (and i'm working as a librarian) : in french, it would ruin all the layout when justification and dialog are involved. The narrow non-breaking space should be use (but it's never used) and as @Semwize said, nnbsp are also streched in kepub...
https://bugs.documentfoundation.org/...g.cgi?id=41652
https://bz.apache.org/ooo/show_bug.cgi?id=23333