There is another username validation in Multisite. It is more strict, as usernames can only contains letters and numbers, so it can not contain underscores, dashes and spaces.
I know, it is not a bug, but I think it would be better to follow WP's own validation, so if it is a Multisite install, than using the function wpmu_validate_user_signup would be preferred.
Do you plan to change this function?