CloudKit fails to sync with CoreData with Constraints

I have a data model with a constraint to ensure an attribute is unique.

When I reinstall the app (with data being available in the cloud), the sync fails with the following error:

2020-07-16 10:33:13.401904+0200 Structured[40519:1639423] [error] error: CoreData+CloudKit: -[PFCloudKitImporterZoneChangedWorkItem applyAccumulatedChanges:error:]_block_invoke_2(440): Failed to save applied changes from import: Error Domain=NSCocoaErrorDomain Code=133021 "(null)" UserInfo={NSExceptionOmitCallstacks=true, conflictList=(
    "NSConstraintConflict (0x600000ab0600) for constraint (\n    day,\n    month,\n    year\n): database: 0xba46bc9a935a60ef <x-coredata://8AB04C93-B9EB-413F-BD0C-9EF854D89D20/Day/p1>, conflictedObjects: (\n    \"0xba46bc9a934e60ef <x-coredata://8AB04C93-B9EB-413F-BD0C-9EF854D89D20/Day/p4>\"\n)"
)}

This error reappears a couple of times and the remote data is never synced and merged with the locally available data.

I'm initalizing the persistance container in the usual way:

    lazy var persistentContainer: NSPersistentCloudKitContainer = {
        let container = NSPersistentCloudKitContainer(name: "Today")
        container.loadPersistentStores(completionHandler: { (storeDescription, error) in
            if let error = error as NSError? {
                fatalError("Unresolved error \(error), \(error.userInfo)")
            }
        })
        container.viewContext.automaticallyMergesChangesFromParent = true
        container.viewContext.mergePolicy = NSMergeByPropertyObjectTrumpMergePolicy
        return container
    }()

I found a similar issue which did not get answered.