APIGateway does not perform request validation when called using POSTMan

Just learning my way through AWS - I have an APIGateway REST API setup with Lambda proxy integration. The API has a model defined, and request validation setup on the body using this model.

Say the model is

{
    "$schema": "http://json-schema.org/draft-04/schema#",
    "type": "object",
    "properties": {
        "propertyA": {
            "type": "string"
        },
        "propertyB": {
            "type": "string"
        },
        "propertyC": {
            "type": "string"
        },
        "propertyD": {
            "type": "string"
        }
    },
    "required": ["propertyA", "propertyB", "propertyC", "propertyD"]
}

Now, if I test the API via APIGateway console, and purposely give an invalid input (omitting a required property propertyD):

{
    "propertyA": "valueA",
    "propertyB": "valueB",
    "propertyC": "valueC"
}

the request fails with the error(400): Sun Jul 11 13:07:07 UTC 2021 : Request body does not match model schema for content type application/json: [object has missing required properties (["propertyD"])]

But when I invoke the same API(and stage) with the same invalid input from Postman, the validation seems to be not happening, and request is proxied to Lambda, which even returns a 200 OK as long as I comment out the parts of code that depend on propertyD.

What's the difference here? Should I be passing in any request header from the client side? I couldn't find anything from the AWS documentations