It appears that overriding the "Application Accepted For Buyer" template will result in emails for this event not sending from the SaaS platform.
They do not show in the "Sent messages" folder in 3scale[1] and do not show up in sendgrid.
There do not appear to be any logs raised in Kibana to indicate the cause of the issue.
Once a template has been overridden there is no way to revert it.
Simply replacing the template with the default text will not resolve the issue.
[1] https://<provider>-admin.3scale.net/p/admin/messages/outbox