Since the update to V6, have issues with some applications (Krusader, mainly, but have noticed in a couple of others - which I just forget at this stage, typical!!), where the Menu Items, and in some cases, icons, are almost invisible due to the colours.
I don’t use Krusader, but this has happened to me with LibreOffice and its icons. It doesn’t correctly pick up all system theme configurations (I believe it has something to do with being a flatpak package), so the icons in my case would just blend in with the background. Look for some sort of theme configuration in settings, either general theme or icon theme.