Bug Can you add information for description?

I find problem that can’t registered user with name from blacklist. Not in Username, but in email address if we don’t have field Username for registration.

I understand why this happens, but it’s not obvious.

In this registration form we don’t have Username, that why WP create user name from email, but if in this black list we have it.

UM plugin prohibits creating a user.

Can you either add a description to the tooltip or, better yet, add a note in the registration form that such names cannot be entered due to the plugin settings? Thanks