Hi Barry,
Have you checked if the issue discussed in the thread provided by AWH84 is the same with yours?
What's the detailed build number of your Outlook?
Please try to start Outlook in safe mode (Press Win + R, type "outlook /safe', press Enter.) and see if the issue can be fixed.
Besides, it is suggested to check for and install all the available updates for both Office and Windows to keep them up-to-date.
If the issue persists, we may try to turn off Automatically group according to arrangement via View > View Settings > Group By... to see the result:

Hope you can find it helpful.
Best regards,
Yuki Sun
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