Maybe a good alternative would be using glimagesink and setting it to win id, which you can get from Qt.
Set Win ID:
gst_video_overlay_set_window_handle(GST_VIDEO_OVERLAY(sink), xwinid);
Get Win ID:
auto winId = videoWidgetContainer->videoWidget->winId();
Just for additional information my videoWidget is of class QWidget.
This works fine for me without any problems, might be worth a try.