text/plain multipart does not use quoting characters
When using Rich Text or Markdown composer, replying to a message will quote the original, but only in the HTML version. The text/plain portion of the message, when viewed by text clients such as Mutt, contain no quoting characters that were not present in the original message replied to. The top level of the quoted message is just mashed together with the text of the new message.
I also am experiencing this issue. Is there any workaround for it?