To reproduce, in a trivial case:

• Bring up a 2.4 standalone
• Insert a user in the admin database with the 'userAdminAnyDatabase' and 'clusterAdmin' roles
• Restart the node as 2.6
• Run the authSchemaUpgradeStep command once, so that the authSchemaVersion is 2.
• Run 'usersInfo' on the admin database. It reports no users.

In more complex cases (i.e multiple users on multiple databases), the command has the same issue of reporting no users when authSchemaVersion is 2. This is, however, the simplest possible reproducing case.

Reproducing script attached.