JIRA Service Desk have a Many (Agents) - to - One (customer) concept. There is no use for this mention to be working when you are entertaining only 1 customer.

Thats true, but in reality it's not only 1 customer in a request... This is why there are request participants and approvers. This is the reason why there is a need for @ mention not only the initial requester.

Regards,
Tim

This is working as expected at this moment, as the mention function does not bring any functionality for customer.

JIRA Service Desk have a Many (Agents) - to - One (customer) concept. There is no use for this mention to be working when you are entertaining only 1 customer. Thus explain why the mention functionality works fro users in service desk team and not for users in customer group. Other customer also does not have the right to access other ticket.

For situation of customer is having JIRA/Software license together (as collaborator), this would be different as they will then be able to mentioned in the ticket. As long as you have seperate group define for this user. Means, for service desk, define them as the normal service desk customer group. Define another group called JIRA SD Customer and assign them to this group. And append the JIRA SD Customer group to the browse project permission to avoid any error thrown as JSD Customer by default are not allow to have this browse project permission

Confirmed this issue is present in version 3.1.4.

I think you have this issue the wrong way round: the problem is that one cannot mention anyone in the Service Desk Customers role, whether the comment is internal or external.

There are many valid use cases where you might want to mention a Service Desk Team member in a comment (particularly an internal comment).

Step 1 is irrelevant - you would not add a customer to a Service Desk project in any other way besides adding them to the Service Desk Customers role (as per Step 2).

In Step 4, this happens for internal and external comments.