Powershell missing array after conversion hashtable to json

Question

Working with Graph API and Intune. I create hash table in my script and covert it to JSON which POST to GraphAPI. But in one case during conversion I lose array. Because of this GraphAPI does not want to accept JSON and returns error.

Case when conversion is correct:

$TargetGroupIDs = @('111', '222')
$AppAssignment = @{
  mobileAppAssignments = @{
  }
}

$AppAssignment.mobileAppAssignments = foreach ($GroupID in $TargetGroupIDs) {
  $Hash = [ordered]@{
    "@odata.type" = "#microsoft.graph.mobileAppAssignment"
    intent        = "Required"
    settings      = $null
    target        = @{
      "@odata.type" = "#microsoft.graph.groupAssignmentTarget"
      groupId       = "$GroupID"
    }
  }
  write-output (New-Object -Typename PSObject -Property $hash)
}

$AppAssignment | ConvertTo-Json -Depth 50

Output:

{
  "mobileAppAssignments": [
    {
      "@odata.type": "#microsoft.graph.mobileAppAssignment",
      "intent": "Required",
      "settings": null,
      "target": {
        "groupId": "111",
        "@odata.type": "#microsoft.graph.groupAssignmentTarget"
      }
    },
    {
      "@odata.type": "#microsoft.graph.mobileAppAssignment",
      "intent": "Required",
      "settings": null,
      "target": {
        "groupId": "222",
        "@odata.type": "#microsoft.graph.groupAssignmentTarget"
      }
    }
  ]
}

But when I have one element in $TargetGroupIDs conversion is not correct:

$TargetGroupIDs = @('111')
$AppAssignment = @{
  mobileAppAssignments = @{
  }
}

$AppAssignment.mobileAppAssignments = foreach ($GroupID in $TargetGroupIDs) {
  $Hash = [ordered]@{
    "@odata.type" = "#microsoft.graph.mobileAppAssignment"
    intent        = "Required"
    settings      = $null
    target        = @{
      "@odata.type" = "#microsoft.graph.groupAssignmentTarget"
      groupId       = "$GroupID"
    }
  }
  write-output (New-Object -Typename PSObject -Property $hash)
}

$AppAssignment | ConvertTo-Json -Depth 50

Output:

{
  "mobileAppAssignments": {
    "@odata.type": "#microsoft.graph.mobileAppAssignment",
    "intent": "Required",
    "settings": null,
    "target": {
      "groupId": "111",
      "@odata.type": "#microsoft.graph.groupAssignmentTarget"
    }
  }
}

Please note difference in brackets after mobileAppAssignments. In first case [], but in second case {}.

Could someone tell what I am missing in second case?