As I said above, the deciding factor is what is meant by syntax. If the request was sent with a Content Type of application/json, then yes, the request is syntactically valid because it's valid JSON syntax, but not semantically valid, since it doesn't match what's expected. (assuming a strict definition of what makes the request in question semantically valid or not). If, on the other hand, the request was sent with a more specific custom Content Type like application/vnd.mycorp.mydatatype+json that, perhaps, specifies exactly what fields are expected, then I would say that the request could easily be syntactically invalid, hence the 400 response.

Missing field can be a field that has not been sent and that's clearly a 400 to me, because the app cannot understand this payload, it violates the contract. IMHO, 400 suits better to not well-formed JSON, JSON that has different key names (contract violation) and JSON that one or more of the field(s) contents is from another type, let's say, you expect a int and got an object. Even not null constraint can be in both status codes, 400 if field not sent at all (and most frameworks understands it as null), and 422 if sent but with null value.