WPF StringFormat not recognised

The StringFormat property - The complete WPF tutorial

R# can't seem to cope with a StringFormat in a WPF binding if the format contains a comma:

<TextBlock Text="{Binding Path=VatReturn.TotalAcquisitionsExVat, Mode=OneWay, StringFormat=£{0:#,##0}}" />

The StringFormat syntax is correct. The code compiles and runs as expected.

String formats that don't use a comma don't seem to be affected.

R# 2018.3.4
VS2017 15.9.9