Why Glyph property in FontImageSource doesn't bind?

Hi community.
I have a strange behavior with FontImageSource. I can't get property binded to Glyph.

<ContentPage.BindingContext>
        <documents:DocumentListViewModel />
    </ContentPage.BindingContext>
    <ContentPage.ToolbarItems>            
        <ToolbarItem Command="{Binding ChangeViewTypeCommand, Mode=OneWay}">
            <ToolbarItem.IconImageSource>
                <FontImageSource Glyph="{Binding ViewTypeGlyph}" Size="26" FontFamily="{x:StaticResource MaterialFontFamily}" Color="{DynamicResource PrimaryColor}"/>
            </ToolbarItem.IconImageSource>
        </ToolbarItem>
    </ContentPage.ToolbarItems>

public string ViewTypeGlyph
        {
            get
            {
                switch (this.ViewType)
                {
                    case ViewType.SmallRows:
                        return MaterialDesignIcons.ViewAgendaOutline;
                    case ViewType.BigRows:
                        return MaterialDesignIcons.CropSquare;
                    case ViewType.Grid:
                        return MaterialDesignIcons.ViewGridOutline;
                }
                return MaterialDesignIcons.ViewAgendaOutline;
            }
        }

I put a break point on the get block and it does not raise.

What am I missing?

Note: setting Glyph to static property works as well

<FontImageSource Glyph="{x:Static icons:MaterialDesignIcons.ViewAgendaOutline}" Size="26" FontFamily="{x:StaticResource MaterialFontFamily}" Color="{DynamicResource PrimaryColor}"/>