Filtering Express object with multiple terms not working as expected?

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If I have a page list and I do e.g. $pages->filterByCollectionTypeHandle(['articlepage', 'newspage']); then the list will contain pages of both those types.

If I have an Express list and I do e.g. $list->filterByConsultantOffice($_POST['office-filter']); where $_POST['office-filter'] is ['London', 'Birmingham'], say, then I only get London offices in the results which isn't the expected behaviour.

Express list is retrieved using $list = Express::getObjectByHandle('consultant');

Am I missing something?

surefyre