Outlook app sync fails

I have an iPhone 20 Pro.

I used the Outlook app for email on the phone. All of a sudden Outlook stopped updating my emails on the phone.

I have an iCloud email account and a Microsoft email account on the Outlook app and my MS email sync without any issues.

It's just the iCloud email that fails to sync.

I did the following:

Reset network settings

Removed and added the email account from the app.

Removed and re-added Outlook from the phone.

Check all the settings related to the email and it's correct.

Restarted the phone several times.

As a last resort I installed Apple Mail app on the phone and that's syncing and updating emails.

Any reason why iCloud fails to update when adding the account to the Outlook app?