I've had the exact same issue with a small local registrar.

Had an account where I managed multiple clients. One of the clients had their "IT guy" contact the registrar for a DNS change. The registrar promptly gave the guy full access to my account, changing the password and locking me out in the process.

As soon as I regained access I moved everything off there.

Don’t be shy. Tell us the name.

This is unacceptable and such companies should change their policy or be out of business.