I have configured kmail to render the mails of some contacts in html (basically some news-feeds) while the general rule is to show plain text only. This usually works well, but sometimes the per contact settings are ignored. 
Sometimes it helps to restart the akonadi service, but sometimes i have to restart kde completely. It also helps (again only sometimes) to add a new duplicate contact with the html settings. After adding this new contact i am able to delete the duplicate (new) contact again and the settings are working again. 

Kmail also randomly asks to create a new user when setting the encryption settings because it is unaware of the already existing one. It also applies for the display of external sources in html messages. 
Therefore, i think this is a general problem of per user settings. Maybe the contact resource becomes unavailable to kmail for some reason. 

Reproducible: Sometimes