Recently I have a seen a question related to pre-header, For example: if someone has entered the pre-header in just 15-20 words then it's taking the extra wording from email.
As per my understanding this is a bug and it should resolved.
It's client behavior, nothing Marketo can control. a mail app is free to take the first 80 printablr chars (or first whatever chars) of text content from the HTML, it doesn't need to look in the first DIV only. Preheader is just a hidden DIV at the top.
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