MSDN says the StringFormat property gets or sets a string that specifies how to format the binding if it displays the bound value as a string.
Since you are using its value, and not displaying it in the multibinding, it will use the value in your MultiBinding StringFormat.
In short: StringFormat on Bindings in MultiBindings will be ignored so to speak.
When you use a MultiBinding, the StringFormat property applies only when it is set on the MultiBinding. The value of StringFormat that is set on any child Binding objects is ignored. The number of parameters in a composite string format cannot exceed the number of child Binding objects in the MultiBinding.
Workaround: expand the MultiBinding StringFormat:
StringFormat="{}{0} 
{1}
{2}
{3}, {4} {5}"
But I guess you could figure that one out as well ;)