The licensing check performed when assigning LICENSED_USER (or another permission that implies the LICENSED_USER) to a group will count users with mixed case usernames twice, if those users belong to a group that is already licensed.

This will manifest itself in the UI with the following error message (or similar):

You cannot grant this permission to group my-group as it would exceed your license limit of 10.

When, in fact, the operation you're performing should not breach that limit.