This has annoyed me for ages and I uninstalled Outlook back in March as I couldn't live with it - that and the dual notification tone every email...
Just reinstalled it to see if the app has improved and this bug was still there. However in the FAQ there is now a fix... by setting the battery optimisation in the settings of my Oneplus 6T to Don't Optimize it instantly resolved itself.
Probably a small hit in battery performance (no other email app requires this, Microsoft why do you have to be so f**kin awkward?) but at least we're there now. It's usable.
I had the same issue that has plagued the MS apps as described in this thread and the suggestion from Chris Sandiford fixed the issue instantly for me. In the Android Settings find the menu where you can specifically disable Battery Optimization on certain
apps and remove the optimization from all the Microsoft ones (Outlook, Teams, Authenticator, Company Portal, etc...). Once that's done reset your phone and viola!
Microsoft, please give higher priority to fixing this issue. The app is very frustrating and you are making a lot of users lose their time looking through every comment in this forum trying to figure out that tricky work around. Also would be nice if MS
android apps wouldn't drain my battery and work properly like all the other email clients!
Also, I have no idea of which FAQs Chris mentioned. If it's very common known issue MS should make sure that it's very clearly visible at least in the forum by pinning it somewhere.