When akonadi fetches mail from my IMAP server it marks all mails as old, which means that mutt will handle them differently just because KMail has fetched them. I tested with akonadiconsole, so it doesn't seem like KMail is doing something special here, and I also don't seem to have the problem when other mail clients fetch the mail, so I don't think the problem is Dovecot either. Reproducible: Always
This bug has never been confirmed for a KDE PIM version that is based on KDE Frameworks (5.x). Those versions differ significantly from the old 4.x series. Therefore, I plan to close it in around two or three months. In the meantime, it is set to WAITINGFORINFO to give reporters the oportunity to check if it is still valid. As soon as someone confirms it for a recent version (at least 5.1, ideally even more recent), I'll gladly reopen it. Please understand that we lack the manpower to triage bugs reported for versions almost two years beyond their end of life.
Just as announced in my last comment, I close this bug. If you encounter it again in a recent version (at least 5.1 aka 15.12; preferably much more recent), please open a new one unless it already exists. Thank you for all your input.