Hi Shawn K. Sheehy,
Welcome to our forum!
Does the issue occur recently in Outlook Desktop Client? Did you have any specific operation before the issue occur?
What is the version of your Outlook? It is recommended that you please check the Outlook client version (File>Office Account>Product Information) and try to upgrade to the latest version to see if the problem is resolved.
To further confirm if the issue is more related with Outlook Desktop Client, it is suggested please log your account in to web version and do the same test to see if the messages exist. (It is recommended to simplify the search criteria and check the search scope: Current Folder、All Folders etc.)
If they are, please refer to the link Incorrect search results when using a date filter in the Outlook search bar - Outlook | Microsoft Learn and have a try to see if it works.
Hope it helps.
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