After drawing the face tag box, Unknown appears in the filtered list. When I type the first letter, the list is correctly filtered. When I type the second letter, the list disappears. When I type the third letter the list comes back correctly filtered. When I type the fourth letter the list disappears. And so on.
Confirmed (version 5.4.0). It would be have been cool to use the feature in practice and add tags to photos before merging the code. If the suggestion of an existing name does not appear, I have to remove one character (or type one more) and hope the suggestion appears. When inserting a new name, the list item to "Add a new person" does not appear. Removing or adding characters would make the name incorrect. I have to press enter (Confirm or Accept) and then later move the tag from tags to persons.
*** This bug has been marked as a duplicate of bug 375375 ***
Fixed with #375375 and not reproducible with 6.2.0