DT ignores charset in Mail on import

I have an e-mail with charset=UTF-8 which I import into DT by dragging/dropping it on a database.

• If the fallback encoding in DT’s preferences is set to “UTF-8”, everything is fine
• if the fallback encoding is set to “ISO-Latin-1”, umlauts etc. are broken (i.e. they are shown as two characters like “Grüße” instead of “Grüße”).

The documentation states:

Fallback text encoding: When decoding an email message, DEVONthink uses the message’s encoding information. When there is no encoding information, DEVONthink uses the encoding that you set in this pop-up menu.

If that is true, why is then the fallback encoding influencing an e-mail with a correct charset setting?