Well, Postfix isn’t going to touch your message body. It just takes an email it’s given, and hands it off to the recipients email server. And then the recipient can read it.
The two variables here are:
If something changed about what’s generating the email itself (if it’s a PHP app, for example, maybe something in PHP changed) – that could potentially be causing the issue you’re seeing. Maybe something regarding the headers isn’t working the same as it had. You may want to review the logs that PHP would write to.
The recipients email client – are you sure the email client and settings here are the same? If if the client wasn’t set to view the email as HTML, it could potentially cause the problems you’re seeing.