The preview strings can also be overridden - you can search for the specific string or .preview and then Ctrl-F search user_notifications to find them.
From the commit message:
For html emails this preview text is hidden with
display: noneto
prevent it appearing within the body of the email. For the plain text
version of the email it will appear within the body of the email.
For which users is this message appearing? It shouldn’t be.
In my email client, I see in the source:
<div class="email-preview" style="display:none"><p>Someone sent you a PM.</p></div>
and it’s hidden in both Thunderbird and Gmail (web).