NativeSize becomes Zero after Switching QGraphicsVideoItem

Assume 1) there are two instances of QGraphicsVideoItem – 'vitem1' & 'vitem2', as well as an instance of QMediaPlayer, 'mplayer'.
2) the signal 'nativeSizeChanged' of vitem1 has been connected to a slot, which records the native size to variable 'nsize'.
connect(vitem1, SIGNAL(nativeSizeChanged(QSizeF)), this, SLOT(nativeSizeChanged(QSizeF)));
3) the mplayer has a valid media resource that can be loaded.

Step 1), vitem1 is bind to mplayer, as "mplayer->setVideoOutput(vitem1);" And it starts loading the media.
Step 2), unbind the vitem1 and bind vitem2 to the mplayer, "mplayer->setVideoOutput(vitem2);"
Step 3), switch vitem1 back, "mplayer->setVideoOutput(vitem1);"

The 'nsize' becomes 0 after step 2 and never be corrected in step 3).

Not sure whether it is a real bug, but please help me check it. Thanks