You may set the RenderOptions.EdgeMode
property to Aliased
on the Grid to avoid this effect:
<VisualBrush x:Name="DissolveInBrush" TileMode="None" >
<VisualBrush.Visual>
<Grid RenderOptions.EdgeMode="Aliased">
...
</Grid>
</VisualBrush.Visual>
</VisualBrush>