Running Fedora 28 KDE spin, all updates current. This has been happening for quite some while, maybe ever since installing Fedora 28.

When some dialogs display, one or more of the buttons is blank. If I hover the mouse cursor over it, it "magically" appears.

Similarly with some dialog boxes, one or more fields will display as blank. If I click into it, the field displays.

Examples shown in images linked below:
LibreOffice Calc, the Sort dialog box, before, the Cancel button is blank. After hovering, Cancel appears.
GNUCash - the Reconcile Information, the Statement Date field is blank. After clicking in it, the date appears.

https://imgur.com/a/yYyIdQN

Thanks for any help,
JimR