Hi!
If this change happened more than a month ago, it could be a synchronization issue between the iOS devices and the Microsoft account.
Completely remove the Microsoft account from Outlook and iOS Settings, then add it again.
- First, sign out of your account in the Outlook app.
- Open your iPhone's Settings > Look for the Accounts, Accounts & Sync, or similar option > Find your Hotmail or Microsoft account > Remove the account.
https://support.apple.com/en-us/guide/iphone/iph44d1ae58a/ios
- Uninstall Outlook (See link below) > Restart your iPhone > Reinstall it from the Apple Store.
https://support.apple.com/en-us/guide/iphone/iph248b543ca/ios
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