Hello,
I have an almost similar issue in creator (for which I created a post on the forum), but with all the components from QtGraphicalEffects, they work at runtime, but I can't get the preview in creator form editor. I also have the little yellow triangle icon warning.
In design studio, it works fine in all aspects. Although there I'm using qt.studio.effects, which in the end is nothing more than a wrapper with Item for convenience of the QtGraphicalEffects.
As I pointed out in my post, I found this bug tracker:
https://bugreports.qt.io/browse/QTBUG-57205
Maybe it can give you a hint.
I'll be following if you find a solution.
Best regards.