Inconsistent reply composition mode (html or text)

When I reply to some mails, the reply is in HTML mode, while to others, it is in plain text mode. I have not configured any domains as html or plaintext domains. I have also selected to send messages out as html and plain text. The choice of reply mode does not seem to relate to the original mail ... e.g. incoming mail was in html, reply composes in plain text. Shift-reply works to change the mode, but I have no way to predict whether I need to shift-reply or reply for html mode.

An added problem is that Quicktext insertions in html mode replies insert the html as plaintext, even though it is marked as html!