When the vendor user is having gmail.com account, the user is unable to login. The error message is "You are not authorized to login with your current credentials. You will be redirected to the login page in a few seconds."

However, Microsoft Support recommends appending mail# to email id and changing Identity Provider to "MAIL". When there is a known workaround, the public documentation should be updated. An automated process can be created too, to automatically change Identity Provider values when gmail.com account is detected, and login failures are detected.

More explanation should be published as well.