Apple HealthKit Sleep Query Nil Value - Why are my sleep samples for last night not present despite the native health app showing they are present?

Question

I am querying Apple HealthKit for sleep analysis using HKCategoryType(.sleepAnalysis) per the documentation. My query is returning samples for the majority of dates and I am performing analysis based on this successfully. However there are instances where the native apple health/sleep app shows sleep samples for a specific date, but when I query the health store directly for the same date range I get nil per the below:

(Example of printing query, results and error to the console)

'''

Query -> HKSampleQuery QoS=Default state=deactivated

Results -> Optional([])

Error -> nil

'''

My Query to the healthStore:

    let startDate = Calendar.current.date(byAdding: .hour, value: -12, to: Date().startOfDay())! // 12 midday yesterday
    let endDate = Calendar.current.date(byAdding: .hour, value: -12, to: Date().endOfDay())! // Midday today



    let healthStore: HKHealthStore = HKHealthStore()

    func fetchSleepDataFor(
        sleepTypes: [HealthKitSleepType],
        startDate: Date,
        endDate: Date,
        completion: @escaping ([HealthKitSleepSession]
    ) -> Void) {
            
        let dispatchGroup = DispatchGroup()
        
        var sleepSessions: [HealthKitSleepSession] = []

        let sleepType = HKCategoryType(.sleepAnalysis)

        let dateRangePredicate = HKQuery.predicateForSamples(withStart: startDate, end: endDate, options: [.strictEndDate])
        
        dispatchGroup.enter()
    
        let query = HKSampleQuery(
            sampleType: sleepType,
            predicate: dateRangePredicate,
            limit: HKObjectQueryNoLimit,
            sortDescriptors: nil
        ) { (query, results, error) in
            
            print(query)
            print(results)
            print(error)
        }

        healthStore.execute(query)
     }

Given I can see the sleep samples and analysis in the native apple health app for the specific date range I would expect them to be present when I directly query them from the healthStore too. I have browsed around the documentation and forums for any mention of this happening but haven't managed to come across anything.

I have performed the following and so I don't think any are the root of the issue:

• Successful request for HealthKitAuthorization from user
• Enabled read and write access to HKObjectType.categoryType(forIdentifier: .sleepAnalysis)

Any guidance as to why this might be the case would be helpful