Bug: Editor: option to remove trailing whitespace doesn't work for C#

My Intendation mode is set to automatic. Code formatting for C# is set to:
* convert tabs to spaces
* allow tabs after non-tabs
* remove trailing whitespace

Now, when editing a C# if I have extra indentation in a line, or lines ending with extra intendation (spaces) these do not get removed.

It appears Xamarin views 'trailing whitespace' as 'remove lines that are only whitespace' i.e. nothing to do with trailing whitespace: https://bugzilla.xamarin.com/show_bug.cgi?id=7719

Ironically, saving files with that option enabled doesn't even remove lines that are 100% whitespace.