Unfortunately this is a pretty old issue that is not related to WebLaF, but to JDK itself.
There was an extended discussion on this issue on GitHub long ago:https://github.com/mgarin/weblaf/issues/130
Shortly - you have enabled custom window decoration which uses shadows/rounded corners - that forces WebLaF to disable decorations AND switch window to non-opaque mode so that it won't paint rectangular background. This makes decoration look nice but the second option actually affects the way fonts are rendered within this window.
I have tried a lot of possible workarounds for this issue but didn't find even a single working one so far. The same issue appears on any JDK version and OS as well.