NSPersistentCloudkitContainer Memory Leak -> Crash? (iOS 15 beta 4 & 5)

Background

I have an established app in the App Store which has been using NSPersistentCloudkitContainer since iOS 13 without any issues.

I've been running my app normally on an iOS device running the iOS 15 betas, mainly to see problems arise before my users see them.

Ever since iOS 15 (beta 4) my app has failed to sync changes - no matter how small the change. An upload 'starts' but never completes. After a minute or so the app quits to the Home Screen and no useful information can be gleaned from crash reports. Until now I've had no idea what's going on.

Possible Bug in the API?

I've managed to replicate this behaviour on the simulator and on another device when building my app with Xcode 13 (beta 5) on iOS 15 (beta 5).

It appears that NSPersistentCloudkitContainer has a memory leak and keeps ramping up the RAM consumption (and CPU at 100%) until the operating system kills the app. No code of mine is running.

I'm not really an expert on these things and I tried to use Instruments to see if that would show me anything. It appears to be related to NSCloudkitMirroringDelegate getting 'stuck' somehow but I have no idea what to do with this information.

My Core Data database is not tiny, but not massive by any means and NSPersistentCloudkitContainer has had no problems syncing to iCloud prior to iOS 15 (beta 4).

If I restore my App Data (from an external backup file - 700MB with lots of many-many, many-one relationships, ckAssets, etc.) the data all gets added to Core Data without an issue at all. The console log (see below) then shows that a sync is created, scheduled & then started... but no data is uploaded.

At this point the memory consumption starts and all I see is 'backgroundTask' warnings appear (only related to CloudKit) with no code of mine running.

CoreData: CloudKit: CoreData+CloudKit: -[PFCloudKitExporter analyzeHistoryInStore:withManagedObjectContext:error:](501): <PFCloudKitExporter: 0x600000301450>: Exporting changes since (0): <NSPersistentHistoryToken - {
    "4B90A437-3D96-4AC9-A27A-E0F633CE5D9D" = 906;
}>

CoreData: CloudKit: CoreData+CloudKit: -[PFCloudKitExportContext processAnalyzedHistoryInStore:inManagedObjectContext:error:]_block_invoke_3(251): Finished processing analyzed history with 29501 metadata objects to create, 0 deleted rows without metadata.

CoreData: CloudKit: CoreData+CloudKit: -[NSCloudKitMirroringDelegate _scheduleAutomatedExportWithLabel:activity:completionHandler:](2800): <NSCloudKitMirroringDelegate: 0x6000015515c0> - Beginning automated export - ExportActivity:
<CKSchedulerActivity: 0x60000032c500; containerID=<CKContainerID: 0x600002ed3240; containerIdentifier=iCloud.com.nitramluap.Somnus, containerEnvironment="Sandbox">, identifier=com.apple.coredata.cloudkit.activity.export.4B90A437-3D96-4AC9-A27A-E0F633CE5D9D, priority=2, xpcActivityCriteriaOverrides={ Priority=Utility }>

CoreData: CloudKit: CoreData+CloudKit: -[NSCloudKitMirroringDelegate executeMirroringRequest:error:](765): <NSCloudKitMirroringDelegate: 0x6000015515c0>: Asked to execute request: <NSCloudKitMirroringExportRequest: 0x600002ed2a30> CBE1852D-7793-46B6-8314-A681D2038B38

2021-08-13 08:41:01.518422+1000 Somnus[11058:671570] [BackgroundTask] Background Task 68 ("CoreData: CloudKit Export"), was created over 30 seconds ago. In applications running in the background, this creates a risk of termination. Remember to call UIApplication.endBackgroundTask(_:) for your task in a timely manner to avoid this.

2021-08-13 08:41:03.519455+1000 Somnus[11058:671570] [BackgroundTask] Background Task 154 ("CoreData: CloudKit Scheduling"), was created over 30 seconds ago. In applications running in the background, this creates a risk of termination. Remember to call UIApplication.endBackgroundTask(_:) for your task in a timely manner to avoid this.

Just wondering if anyone else is having a similar issue? It never had a problem syncing an initial database restore prior to iOS 15 (beta 4) and the problems started right after installing iOS 15 (beta 4).

I've submitted this to Apple Feedback and am awaiting a response (FB9412346). If this is unfixable I'm in real trouble (and my users are going to be livid).

Thanks in advance!