[support] Simplenews removes html headers after update

Hello All, I'm using Simplenews to send out a newsletter. When i create my newsletter issue and send it out (to test e-mail) everything goes fine. After i update my newsletter issue (even not changing any data) and send it out again i receive a plaintext e-mail. Maybe Mimemail treats the updated newsletter issue differently? I haven't got a clue what is going on. Any ideas? Best, Toon