Sometime in the past, our administrators turned "allow unassigned issues" off. Time passed, but the people clamored for unassigned issues. In their short-sightedness, new administrators, instead of turning the setting back on, created an "Unassigned" user (with the username "unassigned").
Fast-forward and the Unassigned user has 1000s of issues assigned. A benevolent administrator is trying to return to allowing properly unassigned tickets. I was able to bulk edit and change the assignment. But, the Unassigned user's "username" is the same as the special username for tickets that are properly unassigned.
I cannot remove the Unassigned user because it appears, to JIRA, to perpetually have tickets assigned to it!
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For anyone else dealing with the same:
I marked the Unassigned user "inactive" so it couldn't be assigned. I changed it's name to "nobody", which helped to see (in some searches) that it really wasn't used. I changed all projects where it was the default-assigned Project Lead to use unassigned properly.