Using New Outlook on Windows for professional communication and productivity
Please go to settings -> account -> sign-in options
Scroll to the bottom for "Use my sign-in info to automatically finish setting up my device after an update or restart"
Turn that option off.
Basically, it doesn't fully close out, so it always comes back at launch. This means that even if you close it on startup, it has already launched and, due to the not closing issue, will never fully close. Lather, rinse, repeat...there it is, next startup.
turn that setting off and it goes away.
This is the correct answer. FINALLY, don't have it auto launching after toggling this setting off. What a terrible design choice to not let people configure this behavior from the app itself. Hopefully they fix/patch it in the future.