It gets even stranger... I can also repro the issue by replacing the MediaPlayer with just a basic MediaElement and (here's the catch) by putting a slider anywhere on the page. For example:
<phone:PivotItem x:Name="itemPlayer" Header="Player View" Tag="Player">
<RowDefinition x:Name="rowPlayer" Height="Auto" MinHeight="240"/>
<RowDefinition x:Name="rowInfos" Height="*"/>
<MediaElement Source="http://download.blender.org/peach/bigbuckbunny_movies/BigBuckBunny_320x180.mp4" x:Name="player" VerticalAlignment="Stretch" Margin="12,0,12,12" />
<Slider Grid.Row="1" VerticalAlignment="Bottom" />
As you can see, the slider in the xaml above isn't even on top of or bound in any way to the MediaElement. Commenting out the slider makes the issue go away.
This definitely seems like a Windows Phone bug and I'd recommend posting it on the
in order to get the right folks to take a look. Also, I haven't searched for a workaround yet so it might be possible to hide the slider or something like that to workaround the issue. If you find a way to workaround the issue in the repro xaml
above; let me know and I'd be happy to help advise on how to apply that workaround to the player framework.