Main Administrator account defaults to 'admin'

We set up a new WordPress website recently in the latest version of WP. The first thing we always do is delete the default user ‘admin’ and create our own.

Last week, our main Admin account username had defaulted back to ‘admin’. The email and password however were still same. Does anyone know why this would happen?

We never did any updates or anything like that. Just tried to log in and had to resend details to our email.

This has happened to our other websites previously and I assumed it was maybe a hack, but I have no idea anymore.