I noticed the same issue. After seeing your post and not being able to find anyone else providing a solution I went into my Amazon account under “Accounts and Lists” choose Customer Service -> help with something else ->memberships, subscriptions or communications -> manage email and mail communications->manage your communication preferences->Email Preferences General Settings ->HTML (text and images).
I don’t know when/why the default was changed but I’m hoping this will solve the issue (though I’ll have to wait for another email from Amazon to confirm it worked)