A disabled user (an user without application access to JIRA) can be chosen as the reporter of an existing issue, simply by editing the reporter and choosing the disabled user.
Disabled users shouldn't be able to be set as issue reporters, since they cannot create issues themselves.
Steps to replicate
- Disable a user (revoking all application access and removing it from all groups)
- Create a test issue with another user
- Start editing the issues reporter
- The disabled user is not even in the users dropdown list to be selected as the reporter
- User is in the list
- User can be chosen and can be set as reporter