Using classic Outlook for Windows in business environments
I tested the same scenario on my side using Classic Outlook, Version 2608 (Build 20326.20132), but the issue did not occur. Emails in the PST were found when All Mailboxes was selected.
In the meantime, you can try the following steps to see if they help resolve the issue:
1.Start Outlook in Safe Mode and test the search again to rule out any interference from Outlook add-ins.
Reference: Open Outlook in safe mode - Microsoft Support
2.Check the Outlook Search indexing settings.
PST files stored in the default Outlook location are normally included in Windows Search indexing. If your PST files are stored in a different folder, go to Control Panel > Indexing Options > Modify and make sure that folder is included in the indexed locations.
3.Rebuild the search index
Go to Control Panel > Indexing Options > Advanced > Rebuild, then allow indexing to complete before testing the search again.
Reference: Fix search issues by rebuilding your Instant Search catalog | Microsoft Support
4.Repair Microsoft 365 Apps
If the issue persists, try performing an Online Repair of Microsoft 365 Apps from Control Panel.
Reference: Repair an Office application | Microsoft Support
Please let me know how it goes after trying these steps.