If writing a message with encryption ( PGP-Mime in my case ) turned on and automatic saving of drafts turned on in the settings, then the drafts get saved in the IMAP/drafts folder as plaintext. I consider this behaviour somewhat counterproductive concerning privacy. It would make sense to either save drafts with my own public key or disable drafting while encryption is turned on!
There is a configuration checkbox for that since 3.9.3, in Preferences/Compose/Writing, just after "automatically save draft", there's "even if mail is to be encrypted".