I am using Mail.app using an imap-account (not Gmail). Just as it should, the app created an 'Archive' folder where it put all the messages in on which I used the archiving function. I was browsing this Archive folder when, out of habit, I used the archiving function again on one of the conversations. This caused the conversation to disappear and I cannot find it anywhere: Not in any folder, the trash, anywhere on the imap server or the Mail.app or its cache.
How can this be intended behavior? Is this a known bug (I could not find any mention of this online). Where are my messages?
I contacted Apple support. This turns out to be a known bug. Look out.