Hi,
Does it show normally on the web mail?
According to my research, this issue may occur if a filter was applied to the default view settings of the Outlook folder. You can verify this on the bottom-left corner of Outlook. Please try resetting view to check the result. To do this, go to View >
Reset View.

If the issue continues, please try opening Outlook in safe mode to check if it behaves normally. Please exit Outlook, press Win key + R to open the Run command, type outlook.exe /safe and then press Enter. This helps eliminate whether the problem lies
in any third party add-ins.
Any updates, please feel free to post back.
Best Regards,
Perry
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