As Microsoft Graph API DriveItem search requests are not accessible when using a Sites.Selected application permission - cf the provided public API documentation: https://docs.microsoft.com/en-us/graph/api/driveitem-search?view=graph-rest-1.0&tabs=http - I would like to propose to unlock these functionalities for this specific permission.
I really think it would be usefull to add the possibility to search for DriveItems within a collection of selected sites.