I have tried both Chrome and Firefox. The same behavior is there too. Same behavior in private mode too. The only way for me to login as another user is to use the "Run as another user" option to launch the browser. While this will work on my desktop I cannot run the Dynatrace service under a user account, it works only as Local System user.
Also the user b@keyman .com can be just anything, does not look like AAD is even using what I entered there. I used psexec to launch the browser in Local System user to replicate the issue.