I eventually raised a TSI for this, and Developer Technical Support indicated that I should file a bug with NSMetadataQueryAccessibleUbiquitousExternalDocumentsScope “since it doesn’t behave as documented”. I've filed that as FB9631965. In the mean-time, I've discovered that NSFilePresenter's documentation describes it as “The interface […] to inform an object […] about changes to that file”, and allows us to monitor imported directories for changes! 🎉 It does, however, have some decade-old bugs that mean the only subitem-related method that actually works consistently is presentedSubitemDidChange(at:). It's a bit of a blunt instrument, but it'll do for my use case.

@Nick: Initialising against the private database as you suggested works on iOS 14.5. It would be really useful if that was in the documentation for initializeCloudKitSchema() if it's not likely to be fixed to support initialising against a public database, to avoid others falling into the trap I did! I've commented to that effect on FB8995024, too. On iOS 15, I can't verify if that's also the case yet because of FB9156476 (the feedback I created during our lab session). For the sake of anyone else who finds this thread, the error that feedback refers to is below. FB9156476 (iOS 15 NSPersistentCloudKitContainer can't sync initial data from server with “Failed to sync user keys”) error: 2021-06-10 21:51:53.672655+0100 MyApp[1230:15921] [error] error: CoreData+CloudKit: -[NSCloudKitMirroringDelegate _performSetupRequest:]_block_invoke(955): <NSCloudKitMirroringDelegate: 0x60000131c180>: Failed to set up CloudKit integration for store: <NSSQLCore: 0x7fa1b1e05f50> (URL: file:///Users/steve/Library/Developer/CoreSimulator/Devices/59435252-2CF1-4980-8ABB-4B46B21385AA/data/Containers/Data/Application/CC6176E7-4FDC-4842-A68F-C6198170CAEA/Library/Application%20Support/private.sqlite) <CKError 0x600002881c50: "Partial Failure" (2/1011); "Failed to modify some record zones"; partial errors: { com.apple.coredata.cloudkit.zone:__defaultOwner__ = <CKError 0x600002805470: "Internal Error" (1/5000); "Failed to sync user keys"> }>

This is still reproducible on iOS 15.0b1 simulators (19A5261u) in Xcode 13.0 beta (13A5154h), with the same code. I've attached the console log I'm seeing on those builds to the aforementioned feedback.