Workflow properties on JIRA permission allow unassignable users to be assigned

jira.permission.assignable.group workflow property does not work correctly. Even if a user is not in the specified group to have an issue assignable to him, he can still be assigned although his username does not show up in the Assignee drop-down list.

Steps to reproduce

1. Create a new user (test) and add him to jira-developers group
2. Copy the default JIRA workflow, add a workflow property to Open step as follows (remember to publish draft):

   jira.permission.assignable.group = jira-administrators

   

3. Ensure Project A is using the default Permission Scheme which has Assignable User permission granted to Project Roles(Developers)
4. Ensure Project A is using the default Roles, where Project Roles(Developers) contains jira-developers group
5. Associate Copy of default JIRA workflow to Project A
6. Create a new issue in Project A and try to assign it to the newly created user (test): the user won't appear in drop-down list but if you type the full username (test) and click Assign button, the user can still be assigned
   
7. Remove the jira.permission.assignable.group from the workflow and publish draft. Try to assign again on the same issue: the user now appears in drop-down list as expected