Ribbon Button tooltip shows two tooltips when VariantSize="Small"

I put a ribbon button inside a Windows Toolbar like below

<ToolBar DockPanel.Dock="Top">
<ribbon:Button Label="Save" VariantSize="Small" ToolTip="This should only be the Tooltip">
<ribbon:Button.ImageSourceSmall>
<BitmapImage UriSource=".\Resources\Images\16\Save_16x16.png" />
</ribbon:Button.ImageSourceSmall>
</ribbon:Button>

</ToolBar>

If you hover it, after around 1-2sec, you will see two tooltips on top of one another. One is the assigned Tooltip and the other is the Label value.