I could never get this to work on my own system with just cache clearing and config resets. I resolved this by changing the Plasma Renderer “Rendering Backend” to “Open GL” instead of “Software”. It seems like sometimes the Plasma Renderer settings widget resets back to the “Software” backend, perhaps on system updates, so I override the system defaults with my own settings like so:
~/.config/kdedefaults/kdeglobals
[QtQuickRendererSettings]
RenderLoop=basic
SceneGraphBackend=opengl