AWS Pinpoint. Mobile push notifications. Why сontent-available always sets to 1?

Question

I am using boto3 and in some cases I need to send silent notifications. I can do it by setting "SilentPush": True. Example:

response = client.send_messages(
    ApplicationId='string',
    MessageRequest={
        'Addresses': {
            'string': {
                'ChannelType': 'APNS'
            }
       },
       'MessageConfiguration': {
            'DefaultPushNotificationMessage': {
                'SilentPush': True,
            }
       }
)

In this case on the device I get push notification with 'content-available': 1, that tells me this is silent notification. The problem is, if I change SilentPush to False, I still get 'content-available': 1. Even if I send notification through aws console, I get the same result. Please help me.