Mail search issue

In Mail.app, search behaves strange: When I type in e.g. "dropbox support", the related e-mails are shown in the left column. But as soon as I select "Persons" or "Header", all search results are gone. E.g., I am looking for all e-mails from John Doe. I type in "John Doe": they're there. I select John Doe as "Sender": all results are gone. The left column is blank.

Is this a bug? Am I the only one with this issue?

Thank you!