Steps to replicate the problem:

1. Create a group named "test group";
2. Create an user named "test" that is not a part of the group above;
3. Create an user named "assignee" that is a part of the group;
4. Add the jira.permission.assignable.group=test group property to a step in the workflow;
5. Create an issue and transition it to that status;
6. Assign an issue to the "assignee" user;
7. Click on assign, type "test" and click on update.

Expected Behavior

JIRA would not allow users that are not a part of the group defined in the property to be assigned to issues.

Actual Behavior

Although JIRA does not auto-complete users in that group, if the username is explicitly typed, JIRA allows the issue to be assigned to the user.