Trello only stores the full name of our users. For compatibility reasons, we attempt to include a derived givenName and familyName in responses for users in locales that typically use Western-order names, but these should not be relied upon to be correct.
Filtering by , , etc. will internally use , and so may not give the expected results. If you know that a name is Western-order, you may find a filter such as (note the trailing space) achieves what you need.