Hi @Cloud_Geek_82 ,
What's the version of your Outlook client(File>Office account>About Outlook), please ensure that you have updated to the latest version.
This started to happen all of a sudden.
Have you performed any specific operations before it(such as change the default account, install any add-ins etc.)?
Based on my research, this calendar peek only shows the events in the default calendar for the default account. I have tried to add 2 accounts and create events in my Outlook 365(version 2010), and found only these events in the default calendar could display normally. So, please first check your default account and default data files in account settings(File>account settings>account settings>Email, Data files).
If you have checked it but the issue still exists, considering the interference of add-ins may also cause it, please try to restart your Outlook in safe mode to test(Windows + R> type"Outlook.exe /safe"> enter).
If your issue has any update, please feel free to post back.
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