Scheduling and managing events using Outlook.com calendar
hi i am an independent adviser and would like to help you resolve your issue
i will suggest you to refer to below article where some basic troubleshooting steps are provided
->https://support.apple.com/en-in/HT204571
from above link
Make sure that iCloud isn’t your default account in Data Files
When iCloud is set as your default account in Data Files, your data won't sync between your devices.
Select File > Info > Account Settings > Data Files.
If iCloud is listed as the default account in the Comments column, select a different account.
Click Set as Default.
i will also suggest you to remove your outlook data file and re-add it again
you can find setting in account section
setting can differ with version but are almost identical
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If issue persist please do let me know and I'll do my best on continuing to help you.