Jira customer can´t be added as watcher or being mentioned

[Anders Grape]

Something has changed in our Jira system recently.

We can´t add a customer as a watcher to an issue. It says "

• The user "XXX" does not have permission to view this issue. This user will not be added to the watch list.

The user also doesn´t appear when we try to mention him in a comment.

The user has browse project access and can see the issue from his service desk portal view. 

I had an older issue where this specific user already was on the watcher list, I then deleted him from the list and tried to add him again which didn´t work...

That´s why I suspect something has changed in the system.

We use Jira Cloud

[Nic Brough -Adaptavist-]

Can the user see the *issue* - not the request in the service desk, but the Jira issue behind it?