Depending on how you login to Jenkins, the role assignment works or doesn't work.
e.g. if you assign "simpleuser" a role, but he logs in as "SimpleUser", the role is not assigned. Same with the other way round.
If the logs out, and logs back in as "simpleuser", it works.
On his /users/simpleuser page, the "Jenkins login name" is, regardless of how he logs in, stated as "simpleuser".