Lockout alert for Capitalized username – bug?

I received an alert that got me worried: “A user, John, has been locked out of (…)” (removed the actual username).

When I checked the Security Log Data I only found failed login attempts for john (no Capital letter; these failed login attempts are more common as it’s for the visible author slug).

But a username of John (starting with a Capital letter) is not the same as john (author slug, all small letters), right? Usernames are case-sensitive, aren’t they?

So is it a plugin bug then that I got an alert for John (with Capital) when the login attempts were for john (without capitals)??

https://wordpress.org/plugins/better-wp-security/