Hi future travellers,
After an investigation, I have finally figured this out. The email template is saved per locale.
Another admin user and myself have set our Discourse locale to UK english, and another admin user has set the locale to default. Our Discourse locale is US english.
When I changed the template, the other admin could see my changes but the emails would still utilise the US locale template. It showed no error when saving, but confusing outcome nonetheless.
The solution was to change my client locale in <domain>/community/forums/u/<user>/preferences/interface to match the server locale which US or set back to default.
Regards,
SS