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.
In countries with a sensible legal system, yes. Truth is not a defense against defamation in all countries. I can't know what country you (generic you) are in, so best to look up your local laws on defamation.
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.