How to get the email of intune device assigned user through Graph API?

I'm using the following API to fetch the managed device properties and it has a field named emailAddress but it is empty

API : https://docs.microsoft.com/en-us/graph/api/intune-devices-manageddevice-get?view=graph-rest-1.0

Response :

{
    "@odata.context": "https://graph.microsoft.com/v1.0/$metadata#deviceManagement/managedDevices",
    "@odata.count": 1,
    "value": [
        {
            "id": "90b67389-2856-43ed-8142-cxxxxxxxxxx",
            "userId": "e99a55d5-fe5d-4142-ad8f-xxxxxxxxxxx",
            "deviceName": "arun.xxxxx_AndroidForWork_7/1/2021_2:34 PM",
            "managedDeviceOwnerType": "company",
            "enrolledDateTime": "2021-07-01T14:34:07Z",
            "lastSyncDateTime": "2021-07-15T07:43:20Z",
            "operatingSystem": "Android",
            "complianceState": "compliant",
            "jailBroken": "false",
            "managementAgent": "mdm",
            "osVersion": "8.1.0",
            "easActivated": true,
            "easDeviceId": "afw21FB64B73D7Exxxxxxxxxxxxxx",
            "easActivationDateTime": "0001-01-01T00:00:00Z",
            "azureADRegistered": true,
            "deviceEnrollmentType": "deviceEnrollmentManager",
            "activationLockBypassCode": null,
            "emailAddress": "",
            "azureADDeviceId": "xxxxxxxx-16fe-4a9e-8bcd-xxxxxxxx",
            "deviceRegistrationState": "registered",
            "deviceCategoryDisplayName": "Unknown",
            "isSupervised": false,
            "exchangeLastSuccessfulSyncDateTime": "0001-01-01T00:00:00Z",
            "exchangeAccessState": "none",
            "exchangeAccessStateReason": "none",
            "remoteAssistanceSessionUrl": null,
            "remoteAssistanceSessionErrorDetails": null,
            "isEncrypted": true,
            "userPrincipalName": "arun.joseph@xxxxxx.onmicrosoft.com",
            "model": "Nexus 6P",
            "manufacturer": "Huawei",
            "imei": "xxxxxxxxxx",
            "complianceGracePeriodExpirationDateTime": "2021-07-15T03:41:27Z",
            "serialNumber": "xxxxxxxxxxx",
            "phoneNumber": "",
            "androidSecurityPatchLevel": "2018-12-05",
            "userDisplayName": "Arun Joseph",
            "configurationManagerClientEnabledFeatures": null,
            "wiFiMacAddress": "xxxxxxxxxxx",
            "deviceHealthAttestationState": null,
            "subscriberCarrier": "",
            "meid": "xxxxxxxxx",
            "totalStorageSpaceInBytes": 0,
            "freeStorageSpaceInBytes": 0,
            "managedDeviceName": "arun.joseph_AndroidForWork_7/1/2021_2:34 PM",
            "partnerReportedThreatState": "unknown",
            "iccid": null,
            "udid": null,
            "notes": null,
            "ethernetMacAddress": null,
            "physicalMemoryInBytes": 0,
            "deviceActionResults": []
        },
    ]
}

Note:

1. I have redacted some values with xxxx for obvious reasons

2. I have enrolled this device using company portal android app

As u can see, the emailAddress field is empty but userPrincipalName field is giving the user's mail id.

but I'm not sure if userPrincipalName will always have the email of the mapped user.

My questions:

1. Will userPrincipalName  field always have mapped user's email?

2. When will the emailAddress field be empty and when will it have value? 

Kind regards,

Arun Joseph