[Bug] Slider ThumbImageSource: Binding not working for Glyph

Description

The value binded into

            <Slider.ThumbImageSource>             
                <FontImageSource FontFamily="{DynamicResource MaterialFontFamily}"
                             Glyph="{Binding IconFont}"
                             Size="38"
                             Color="Red" />            
            </Slider.ThumbImageSource>          

is ignored. If you use explicit value for Glyph it works

            <Slider.ThumbImageSource>             
                <FontImageSource FontFamily="{DynamicResource MaterialFontFamily}"
                             Glyph="&#xf11f;"
                             Size="38"
                             Color="Red" />            
            </Slider.ThumbImageSource>     

The value of IconFont is set in the correct way...If I add a Image with a Glyph Binded value, the bind works as expected

                <Image.Source>
                    <FontImageSource
                        FontFamily="{DynamicResource MaterialFontFamily}"
                        Glyph="{Binding IconFont}"
                        Size="20"
                        Color="#999999" />
                </Image.Source>    

Steps to Reproduce

I attach a project. A list with 3 row. In each cell I have:

• 2 slider. The first one is with binded Glyph value (not working) The second one (disabled) has a Glyph value working without Binding An image, with the same bind value of the first slider, and it works as expected

In the sample project move the slider to change the Glyph value

Expected Behavior

Glyph should appears if binded to a value

Actual Behavior

Glyph doesnt appears

• Version with issue: x.f. 4.0 sr3
• Last known good version: none
• IDE: VStudio for mac

Does not work on ios and android (both)

BugSlider.zip