Rekognition.detectLabels: Face Label missing instances

Hello,

What is the meaning of the below output from Rekognition.detectLabels (Management Console) when the picture contains many clear, big faces, but the AI detects no instance ? Does the instance array contains the detected faces, or does it have another meaning (where can I found the specification for the Rekognition.detectLabels.DetectLabelsResult) ? On same picture Rekognition.indexFaces detects some faces.
In SDK the specification for Rekognition.detectLabels.DetectLabelsResult.getLabels.getInstances is:

If Label represents an object, Instances contains the bounding boxes for each instanceof the detected object. Bounding boxes are returned for common object labels such as people, cars, furniture, apparel or pets.

So, shouldn't the instances contains bounding boxes, as long as Face is a bounding box Label ?

		{
			"name": "Face",
			"aliases": [],
			"parents": [
				{
					"name": "Head"
				},
				{
					"name": "Person"
				}
			],
			"instances": [],
			"categories": [
				{
					"name": "Person Description"
				}
			],
			"confidence": 99.99898
		}

Thank you,
Mihai ADAM