You example of the mask token are as follow
XAML

<StackPanel HorizontalAlignment="Center" VerticalAlignment="Center">
    <TextBlock Text="Mask: A" />
    <telerik:RadMaskedTextInput Margin="0,5,0,10" Mask="AAAAAA" />
    <TextBlock Text="Mask: a" />
    <telerik:RadMaskedTextInput Margin="0,5,0,10" Mask="aaaaaa" />
    <TextBlock Text="Mask: L" />
    <telerik:RadMaskedTextInput Margin="0,5,0,10" Mask="LLLL" />
    <TextBlock Text="Mask: l" />
    <telerik:RadMaskedTextInput Margin="0,5,0,10" Mask="llll" />
    <TextBlock Text="Mask: \" />
    <telerik:RadMaskedTextInput Mask="\#\\###" />
    <TextBlock Text="Mask: Any other characters" />
    <telerik:RadMaskedTextInput Mask="N\ame: llll" />
</StackPanel>

and the result should be as follow
Mask A = Aa1 +;
Mask a = Ab-,"

which is fine.
But if I use any of the capital A or L, in the MVVM, the binding is not working anymore.

I have attached a file.
Run the program and press add button
Enter info in First Name, then enter info in Last Name.
Put a break point in the file RecipientData.cs at the properties FirstName on get and set, and do the same for LastName properties.
In the xaml, the only difference between those 2 controls one the Mask="&gt;L1A29" for the first name and Mask="&gt;l1a29" for the last name.  I do not know what is going on behind the scene of this control, but I know there are some issues.

REASON FOR DECLINATION
This is expected behavior in the MaskedInputControls. Please read more in the comments.