Unable to unassign account with no target reference

1. import resource (e.g. localhost-dbtable-advanced-nosync.xml)
2. Create user
3. Assign account to the created user -> select the imported resource
4. Go to Resources -> List resources -> delete dbtable resource
5. go to Users -> List users -> open Edit page for created in the step 2 user
Result: in the Assignments sector " (no target) Show more" message is shown
6. select check box for the assignment -> select Unassign from the menu -> click Save button
Actual result: users list page is opened, Success Save user message is shown
7. Open the same user
Expected result: assignment is unassigned
Actual result: assignment is still present