On Spring Webflux, invalid json request results in HTTP 500

Describe the bug
On Webflux, attempting to POST invalid JSON results in an HTTP 500.

To Reproduce
POST this request, setting Content-Type:application/json:

"false":true

This returns the following 500 response:

{
    "errors": [
        {
            "code": "INTERNAL_ERROR",
            "message": "The service experienced an internal error. Please try again later.: [details: Cannot construct instance of `graphql.kickstart.execution.GraphQLRequest` (although at least one Creator exists): no String-argument constructor/factory method to deserialize from String value ('false')\n at [Source: (String)\"\"false\":true\"; line: 1, column: 1]]"
        }
    ]
}

Expected behavior
The HTTP response should be 4XX (probably 422). Also the error verbiage should obfuscate the internal ObjectMapper error with something more generic (e.g., 'invalid JSON').

Desktop (please complete the following information):

• OS: Mac Big Sur 11.1
• Postman 8.0.3

[BlasiusSecundus]

I think that according to the specs (https://github.com/graphql/graphql-spec/blob/main/spec/Section%207%20--%20Response.md#errors), instead of HTTP 400/422, a valid GraphQL error response should be returned if the request is invalid.