New users not showing up in Post -> Edit

Background: A client has used first and last names as the user names, and we are trying to replace the old users with new users with more secure user names. We need to assign the posts for the old users to the new user accounts.

After we created a new user, we went to Posts -> Bulk Actions -> Edit. The list of users does not show the new user. I have never seen this before. How do I force that list to recreate from the actual user list?

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s_ha_dum’s answer prompted me to look into this a little further. I checked the users in the database. It turns out that the dropdown list in Posts -> Edit is using the Display Name, not the User Login, as I had expected. So, the problem was not that WordPress was functioning incorrectly, but that my understanding of what field was being used was incorrect. I guess this was a “false negative.”