Contextual filter exclude ignored

I'm using Views Field View that passes a view with a contextual filter set to exclude, like so:

View 1:
-field a
-field b
-contextual filter field b > set to exclude

View 2:
-field b
-views_field_view field
--view = View 1
--argument = field b

When I test View 1 with an appropriate field b value, I get expected results - the list excludes field b items.
When I test in View 2, I get the reverse - the list only shows field b items, and excludes everything else.

Looks like the exclude option in the first view is being overridden or isn't being respected when I pass it through the second view.