Not sure if it is related to https://bugs.kde.org/show_bug.cgi?id=395711, but since KMail 22.08 / Framework 5.97 mails with HTML/External content are not opened by default in HTML and with ext. content, although this is specified for the mail sender in the address book Same for preview as well as mail opened in separate window
*** This bug has been marked as a duplicate of bug 458245 ***