Form submissions including HTML in email notifications

I have a website where the core contact form is submitting the form markup along with the content.

So for example, this is in the email notifying the site owner of the submission:

<div class="field__label">The sender's name</div>

          <div class="field__item">Williamjeply</div>

<div class="field__label">The sender's email</div>

          <div class="field__item"></div>

<div class="field__label">Subject</div>

          <div class="field__item">How negotiations work! A must read  book</div>

<div class="field__label">Message</div>

          <div class="field__item">LISTENEVERYHOW – How Negotiations  Work, is an easy-to-read book and pragmatic approach to get the best results  from a negotiation. It is adaptable in content and style – as a story book  for leisure readers, life skills manual for entrepreneurs and  professionals, or even as a business school handbook. <br />&#13;  <br />&#13; This book is written with clarity and easy eloquence of a man who knows what  he is talking about, telling captivating stories in well sequenced chapters  that all end with enticing nuggets. A rudiment skill in negotiation is  listening and the major themes of this book are reflective of the insights that  make the difference in negotiations. <br />&#13;  <br />&#13; You are one click away from having this excellent book for just $5.99</div>


I’ve made sure the permission allowing HTML emails is checked in the SMTP settings and there are no other obvious errors.
Does anyone have any clues why this is happening?
The issue queue doesn’t provide much help on this.

The site was recently upgraded to 8.9.12 and is just using the core contact form.

Thanks in advance.