After upgrade to Windows 11, Outlook searches return no results. This issue will happen with any account where the emails and other items are stored locally in PST or OST files such as POP and IMAP accounts. For Exchange and Microsoft 365 hosted accounts, this issue will affect offline search for the data in the locally stored OST files.

https://support.microsoft.com/en-us/office/outlook-search-does-not-return-results-after-upgrade-to-windows-11-cc54dfee-ee78-4123-a9af-7986748052ef