Hi,
Please try going to View > View Settings > Group By, make sure the group by is set to "None", or you can click "Clear All" to clear all Group settings:

If the issue continues, it is suggested to close Outlook, then try the steps below to check the result:
1. Press Win + R.
2. Type "outlook /cleanviews", press Enter.
Hope the above methods can be helpful.
Regards,
Yuki Sun
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