The DataContext is not distribute to Large and Small contents in some cases.
Possible workarouns:
1) Declare the DataContext of TileView in xaml, for example:
<Window.DataContext>
<local:MainViewModel />
</Window.DataContext>
2) Assing DataContext Binding to the three contents:
<telerik:RadFluidContentControl.LargeContent>
<Grid DataContext="
<telerik:RadFluidContentControl.Content>
<TextBox FontSize="16" DataContext="{Binding}"
<telerik:RadFluidContentControl.SmallContent>
<TextBlock FontSize="12" DataContext="{Binding}"