Changing an issue's milestone to "None (backlog)" removes it from "None (backlog)" filter
Created by: baldyeagle
If you change an existing issue's milestone to "None (backlog)" and try to filter by that milestone on the issue list, the reassigned issue will not appear.
I think that action sets the milestone id to null, as changing the query string of the filter list URL from "&milestone_id=0" to "&milestone_id=null" makes the issue appear.