I recently ran into this issue on my test machine, turns out it was because I was using the same password for the reseller/root account as the user account.
With cPanel/WHM, you can login with the username for that account but the password for the reseller in charge of that account (in your case, the root user). Doing this will log you in as a reseller, but default your cPanel view to that account.
Use different passwords for your user accounts than your root/reseller account and you'll be able to log in as the user

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