2.6.2 When writing a post “Preview this post” shows wrong charset

When writing (not editing) a post, the “Preview this post” button shows the post utf-8 encoded while everything is set to iso. This means characters like é will be shown as é in the preview.
Saving posts is done correctly, so I guess my settings are correct for using iso. Is there a way to prevent the preview from converting characters to utf-8?