Shoch, thank you for your reply.
It appears that the issue lies with the Outlook Mobile app, as it is not syncing with the calendar on Outlook web. This may be due to a problem with the Outlook mobile app on your iPhone.
- Go to your iPhone or iPad's Settings > Accounts & Passwords > select the account you're trying to sync.
- Make sure "Calendars" and "Contacts" are enabled and then check your sync again. If it works, you're done!
(If it is already enabled, disable the toggle and toggle them again)
If that didn't work, remove your account on the Mail app and add it again.
Select Outlook > Click Delete account.
Note: Since the Outlook calendar contents are not synced properly, your changes might not reflect sometime after readding the account on the Outlook mobile. so, proceed with caution and save your data.
I hope this information helps. If you have any questions, please let me know and I will be happy to help you further.
Best regards,
Gunasekar N