[REG:5.15.7->5.15.8]: repaint() on a widget makes QGraphicsOpacityEffect apply multiple times

When using a QGraphicsOpacityEffect on a widget (with for example opacity of 0.7), repainting the widget makes the graphics effect apply multiple times (enterEvent and leaveEvent repaints in the sample application). Disabling/Enabling the graphics effect resets this stacked graphics effect rendering (focusInEvent and focusOutEvent in the sample application).

Attached videos about working (5.15.7) and non-working (5.15.8) situations.

Attached sample application (based on Qt's MDI example) reproduces the issue, reproduce steps can be seen in the videos.