Plain-text email incorrectly contains HTML entity escapes

When I receive email notfications about e.g. pull requests, they are rendered with &amp; &lt; &gt; etc instead of the correct plain & <> characters in the actual text.

Here’s a screen shot from the FastMail app on IOS for the notification of Charcoal-SE/SmokeDetector PR #6780

My email client preferences are set up to prefer plain text over HTML.

Possibly related: Plaintext mail on issue update

(Bewilderingly, this web page silently translated &amp; into a literal & character, etc; I had to go back and add `&amp;` code formatting after submitting.)

it is maybe on the side of IOS, they admitted that here,

https://developer.apple.com/forums/thread/681023