When a Project has the Current assignee or the Reporter in the Assignable User Permission, JIRA is allowing users that don't have the Assignable User permission to be assigned.

Steps to Reproduce

1. Make sure the Permission Scheme tied to the project has the Current assignee (or the Reporter) in the Assignable User permission.
   
2. Create a new issue and set the assignee as a user that does not have the Assignable User Permission.

Actual Results

• When entering the first letters of any user, this user shows in the user picker list even though this user does not have the permission to be assigned any ticket
  
• The issue is created with a user that should not be assignable.

Expected Results

The user should not show up in the Assignee field.

Notes

This appears to only occur during Issue Creation. Once an issue has already been created, JIRA will not allow a non-assignable user to be set as the assignee, as shown in the screenshot below:

This can be observed by following these steps:

1. Create a new issue with a non-assignable user per this bug
2. Change the assignee of the newly created issue to an actual assignable user
3. Change the assignee back to the same non-assignable user from #1. JIRA will not permit this, and this bug does not occur here.

Workaround

Remove the Current Assignee (or the Reporter) from the permission