I have a view embedded into an email with a custom theme for the view. Since the theme that sends the email is the admin theme, mime mail looks for the view theme override in that admin theme, instead of the theme specified under "Theme to render the emails" in the mail system settings page. It finds mimemail-message.tpl.php in the correct theme folder, not the admin which sends the email.
Mail System - 7.x-2.33
Mime Mail - 7.x-1.0-alpha1+9-dev
Simplenews - 7.x-1.0-beta2+10-dev