Summary
No 'Resolved issue' notification is sent, if the transition in the SD project has:
- A screen associated with the 'Resolution' field
- A post-function setting the field
Even if the ticket did not have previously a resolution set, if it's changed twice, the notification is not sent. For example, if the ticket was 'Unresolved', then transitioned to 'Done' via screen and to 'Declined' via post-function, the issue's history will show:

As though the issue was not initially Unresolved, as we don't see an entry from 'Unresolved' to 'Done' first. The notification is not triggered.
According to the documentation, the customer should receive a notification when the issue resolution is set:
When a customer submits a request, they receive email notifications as the issue reporter when:
the request is created
- a comment is added to the request
- another participant is added to the request
- the request is resolved with a set resolution field
- the request is reopened with a cleared resolution field
Steps to Reproduce
- Create an SD project
- Create a transition that resolves the issue
- Create a post-function to set the 'Resolution' to some value, such as:
The Resolution of the issue will be set to Declined.
- Associate a screen to the ticket with the 'Resolution' field included
- Create and resolve the issue
Expected Results
User is notification for both 'create' and 'resolve' transitions.
Actual Results
Only 'create' in this case will trigger notification. No notification for when 'Resolving' the issue. This is also preventing Customer Satisfaction to be sent.
Workaround
Remove either the post function or the field from the screen (or the screen completely from the transition).