Naming convention for theme templates not working

Problem/Motivation

When trying to override the email template using theme file this doesn't seem to work.

Steps to reproduce

Following the instructions in the readme file and copying the twig file to the theme under templates folder and keeping the same simplenews-newsletter-body.html.twig filename to incorporate all newsletters this doesn't seem to work.

I have Symfony Mailer installed to be able to send HTML mail and I have changes it's settings to use the same theme to render the mail as what we are using so I'm sure that setting isn't an issue.

The only way I seem to be able to override the email template is if I rename the twig fie to "email--simplenews-newsletter-body.html.twig" but then I'm guessing Simplenews doesn't pick this up at all as the content from the Newsletter issue that I use for testing isn't then sent in the email, although the changes to the template I made in the twig file are present.

Proposed resolution

I it the naming scheme that is wrong here, or am I missing something obvious? All the topics around the same or related issues seem to be from years ago when the template files were TPLs so apologies for asking this if it has already been addressed, but I have very much hit a wall here.