It’s not Monzo’s problem. Try the same thing with another bank - chances are you’ll see the same thing as that’s what the regulator (the FCA) tells them to do.
If a business is a sole trader then they could have either: a personal account or a business account.
If a business is limited company then they’ll need to have a business account.
You can probably tell which one it is by the account name given to you by the business. If it sounds like someone’s name, it’ll be personal and if it sounds like a business name, it’ll be a business account.