Bulk clearing "assigned to" on issues has no effect

Steps to reproduce:

1. Go to the issues page of a project
2. Tick the check-boxes for one or more assigned issues.
3. From the 'Assignee' drop down above the list, select 'Unassigned'.
4. Click the 'Update issues' button.

Expected: The ticked issues are now unassigned

Observed: There is a message that says 'X issues updated' but no changes are actually made. The issues are still assigned.

Version: 7.5.3 omnibus community edition.