QWidget with border-image

I have my own widget (MyWidget, inherits QWidget) with the background (I use StyleSheet). In the main window, I want to add a few instances of MyWidgets. However, when adding promoted widgets from MyWidget, the background is lost, but the other formatting from css is preserved. I googled that QWidget ignores the background for optimization purposes, so in the MyWidget c-tor I call setAttribute(Qt::WA_StyledBackground), but this does not affect.

How do I make widgets that are promoted from MyWidget keep the background?