SUMMARY
In KMail's settings under "security" I have NOT checked "Prefer HTML view over plain text". I even went into the folder preferences for my inbox and specifically set it to prefer plain text over HTML, yet many e-mails are displayed as HTML by default. 
After opening the mail I can manually (and only temporarily) switch to plain text view, but I'd like to force that as the default for all mails.

STEPS TO REPRODUCE
1. Make sure that HTML view is NOT preferred in the global security setings
2. Optionally, also set the inbox to prefer plain text view.
3. Open some mails from various sources, some of them will still open in HTML view

OBSERVED RESULT
Some mails open in HTML view even though plain text view is preferred

EXPECTED RESULT
When HTML view is not preferred (when plain text is preferred), ALL mails should be forced into plain text view by default when opening them.

Also, it might be less confusing if in KMail's security settings there were radio buttons for selecting either "prefer plain text" or "prefer HTML" like in the folder settings, instead of just a check box for "prefer HTML view"


SOFTWARE/OS VERSIONS
Operating System: Arch Linux
KDE Plasma Version: 5.22.5
KDE Frameworks Version: 5.86.0
Qt Version: 5.15.2