(Just in case it helps: I had a quick look at my setup, and didn’t see a problem. So I’d wonder if you have some theme or plugin which affects your setup. What I did was to visit mail-tester.com to get a temporary destination, then use Admin->Emails->Preview Summary to send a summary to the temporary destination, and then click through on mail-tester to view the HTML and plain versions. It might be worth trying the same tactic to see if anything is any different for you.)