Split off from, but related to, https://tracker.phpbb.com/browse/CUSTDB-826
Received an email with 🖌️ 📐 in the subject line, where the actual topic title has 🖌️ 📐.
Email subject lines have their own means of encoding non-ASCII text, which is unrelated to HTML entities, so HTML entities should never be appearing in that context.
The email template was `topic_notify.txt` (template variables injected by `phpbb/notification/type/post.php`).
PR up shortly.
(Please note if quoting in a reply, posting actual emojis will break Jira, so you need to double-encode them, i.e. it will look like &#128396;️. And what I just typed there was &amp;#128396;️. And that was &amp;amp;#128396;️... and so on ad infinitum).