Notification emails not being sent, are in plain text, but rich text selected?

We’re seeing an issue with various notification emails from JM not being sent. They’re hitting the POST SMTP plugin for sending, and it’s failing to send, with an error of “This email has empty body”.

However when I resend the message manually (via POST SMTP resend function), I can receive the email – the HTML is printed as plain text, and the headers are showing as:

Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: quoted-printable
Content-Disposition: inline
MIME-Version: 1.0

We have “Rich Text” selected in the JM settings.

Is the incorrect content-type is being set somewhere?

WP v6.4.3. Job Manager v2.2.2