If I remember correctly, clicking the "Rich Text" button at the top and hence disabling HTML composition used to cause this preference to be saved for future e-mails. Further, I think "Rich Text" was turned off by default. Composing a new mail, "Rich Text" always seems to be re-enabled, and I always have to re-disable it manually, as I prefer to send plain-text e-mails. This setting should either be saved or configurable in kmail's options. Looks like a regression.
(I'm using kmail2 5.6.3, which is not listed in the selection above.)
it's a bug in 5.6.3 fixed in 5.7.0