I’ve gone back to Ubuntu and now creating a bibliography works with JabRef 5.5 (and LibreOffice is now 6.4.7.2). When I used a blank document to test the connection to JabRef, the bibliography was generated right under the first reference.
I’ve also managed to get the bibliography to work with my production database. The bibliography still updates having moved it to the end of the (existing) document.
Thanks for your patience. I have no idea what happened on Friday…