Using Outlook for iOS for personal email and calendar management
So glad someone else has this problem!
I have re-installed all of the Microsoft apps I've previously used to see if any others are effected.
I have this exact same issue with Teams, One-Drive and Outlook.
All other Microsoft apps seems to be okay though.
There doesn't seem to be any issue if I do a factory reset and just install the app, but if I use my full IOS restore then the issue comes back.
So as mentioned - could be due to large dataset due to calendar etc.
I've been dealing with this issue in teams for several months now, the issue goes away after a few weeks until I change my password again (corporate sign-in)