Update: "Preliminary root cause: A portion of third-party managed network infrastructure that facilitates authentication requests and access to Microsoft 365 services was degraded."
(Emphasis mine.)
(Emphasis mine.)
You can buy connectivity from 5 diverse providers per data center, but if one of them continues to advertise your routes while not being able to actually pass traffic to your destination, you’re “down” to some people through no fault of your own.
I’ve had this happen with AT&T, XO/Verizon, and Cogent in recent years. It’s not uncommon. In all cases it was mis-configured ISP routers. Usually a support engineer admits a QoS or DDoS-protection configuration went wrong.