Issue assignee not updating on sidebar

Summary

When adding or removing an assignee on the sidebar, the assignee UI is not updated.

Steps to reproduce

1. Add an assignee on the sidebar
2. Apply the change by clicking the 'Apply' button
3. Add another assignee on the sidebar
4. Apply the change by clicking the 'Apply' button

Notice the second assignee is at first displayed, then disappears. Refreshing shows system notes as the assignee being added and removed.