In order to keep this post short, allow me to point you to a rather old Scribus bug report, which perfectly describes the problem. This specific bug has been fixed upstream, and is not present in Ubuntu 18.04.1, or Xubuntu 18.04.1, or Linux Mint 19. BUT, it is present in elementaryOS 5.0 which is very strange. It would be nice if someone provided some solution since it cripples an otherwise perfectly working package:
https://bugs.launchpad.net/ubuntu/+source/scribus/+bug/177805
Kind Regards