I installed Fedora 26 KDE when it still was beta. The problem is simple: Libreoffice can't find the nimbus fonts. I do have installed on my system (I see them in the font management application), but I can't use them in Libreoffice because they do not appear in the font menu.
Users with many fonts may find it useful to only have certain fonts enabled (or active) at certain times. To facilitate this, this module will allow you to disable individual fonts, or whole groups of fonts. Disabling a font does not remove the font from the system, it simply hides it so that it no longer appears within applications. Re-enabling a font will then allow it to be used.