on Kmail 5.13.3 (Kubuntu 20.04) :

when i right-click a sender's email address in the message reading window, there is an option to always show HTML format from this sender, with a check-box to the left of it.

when i click this option, and then navigate to it again afterwards (by right-clicking on the email address again), it still appears unchecked.

sometimes i have to try several times before the option is confirmed by the check-box being blue.

Is this just because there is a significant delay in Kmail 'remembering' the option?

(There is nothing wrong with the mouse!)

Please check the settings for this sender in your address book first...