If you’re using system users, as you should be in a Virtualmin system, you should use the system passwd command and not create a separate user password just for Webmin.
At least, I don’t think webmin passwd resets system passwords.
Webmin can have an independent password for your users, but you generally shouldn’t, because one day you’re doing to be confused when you can log in to ssh and not Webmin or vice verse when using the same password…it’s because you split the passwords by setting a new Webmin password instead of sticking with the system password.
We should probably make webmin passwd warn about that duality.