Microsoft has identified the root cause and hopes to have rolled out a fix by Tuesday 11th February 2020.
"We've completed testing a solution to mitigate impact for affected users. Once implemented, this fix should allow users to receive new mail notifications without performing a page refresh. Furthermore, we’re working to enable expanded logging in a portion of the affected environment to provide additional data on the root of the problem. We’ve initiated deployment of this change throughout the affected environment, which we anticipate will complete by Tuesday, February 11, 2020."