I've come across a problem with a user creating a Story/Bug/Task underneath an Epic in that when they do, Jira Cloud does not retain the value in the Epiclink field. The user in question is a member of an 'External Users' group. The problem does not occur when doing the same thing with a user that is a member of the 'Internal Users' group.
I have checked the permission scheme for the project and both internal and external groups have access to the same permissions, in fact the external users group has slightly more. Global Jira permissions have been left at their default and neither group has been specifically added.
I don't think it should have any effect on the scenario but Issue Level security has been set up so that external users can only see the Epics which relate to them. The external user can see the Epic which they are trying to link to when creating their Story/Bug/Task. However, when they first create their issue they cannot see it because the default security level is set so that internal. However, I'm planning to use automation to get by this and I'm only mentioning it for completeness.
What else might be causing the Epiclink value to disappear after issue creation by external users?