Hello,
Welcome to Centrify Community!
From your description, it seems your Mac has FileVault 2 enabled.
If that is the case, only FileVault 2-authorized users will be shown, because they are the only users authorized to open the start up disk.
To check whether your Mac has FileVault 2 enabled, you can run the following command on your Mac:
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fdesetup status
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Hope this helps.
Best Regards,
Jason