Microsoft and other large companies are made up of many smaller groups, and to make this work there are a lot of extra rules. For the most part those rules serv

Microsoft and other large companies are made up of many smaller groups, and to make this work there are a lot of extra rules. For the most part those rules serve a purpose and the benefit outweighs the cost. Sometimes these rules create extra problems, because of how people actually make choices and use the resulting system. It is likely there is a detailed change approval process for updating a *.microsoft.com DNS record. Or that only certain depts can make changes, and only for certain reasons. Someone involved with Teams avoided that by using a separate domain and a cname record, then they could update their separate domain records more easily. Later someone forgot to register this domain, and/or forgot to update the DNS to somewhere that is over their control, or forgot to put in sufficient checks to prevent tokens being shared with unauthorized servers.