Hi
yes this is exactly that, if you put something, once saved, the subject is now back to the full html code of what is in the template.
I use JCE editor, but same behaviour with others editors.
It's most like the subject input value is always replaced with the html code.
I've also tested with a simple "hello" in the mail body and same issue, the subject become "<p>hello</p>".
<table border="0" style="padding: 10px; background-color: #eeeeee; font-family: Arial; font-size: 11px;">
    <tbody>
        <tr><td>
                <table border="0" style="background-color: #ffffff;">
                    <tbody>
                        <tr>
                            <td style="color: #888888; padding: 10px;">
                                <h1 style="color: #888888;">
                                    <img src="images/mini-logo.png" alt="mini logo" dir="ltr" loading="lazy" /> Site Name</h1>
                                <hr style="color: #8fc400; background-color: #8fc400; height: 5px;" /></td>
                        </tr>
                        <tr>
                            <td style="padding: 10px;">
                                <p>Bonjour,
                                </p>
                                <p>veuillez trouver ci-dessous votre coupon de réduction d'un montant de <strong>{coupon_value} HT</strong>
                                </p>
                                <p>Voici le code à utiliser dans votre panier :
                                </p>
                                <p><strong> 
                                        <span style="font-size: 18pt;">{coupon_code} 
                                        </span></strong>
                                </p>
                                <p>Vous pouvez soit :
                                </p>
                                <ul>
                                    <li>Le renseigner dans votre panier en cours dans la case <strong>N°5 "Remise chèque boutique"</strong> puis cliquer sur enregistrer pour appliquer.
                                    </li>
                                    <li>Ou cliquer sur votre menu <strong>"
                                        <a href="mes-coupons" style="text-decoration: underline;" title="Cliquer ce lien pour accèder à vos coupons sur votre compte">Mes coupons</a>"</strong> puis cliquer sur le bouton bleu <strong>"Utiliser ce coupon"</strong> pour directement l'appliquer à votre panier en cours.
                                    </li>
                                </ul></td>
                        </tr>
                        <tr>
                            <td style="background-color: #505050; color: #c8c8c8; padding: 20px;">
                                <p>Service Client <br />10 Rue des Balles, Paris - Ile de France - France 75011<br />01 23 45 67 89
                                </p>
                                <p>Copyright © 
                                    <a href="https://www.example.com/">The website name</a>
                                </p></td>
                        </tr>
                    </tbody>
                </table></td>
        </tr>
    </tbody>
</table>