Ideal Scenario for 422: In an ideal world, 422 is preferred and generally acceptable to send as response if the server understands the content type of the request entity and the syntax of the request entity is correct but was unable to process the data because its semantically erroneous.

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.

Your answer (422) makes sense to me. This is also what Rails (respond_with) uses when a resource couldn't be processed because of validation errors.

Michael Kropat put together a set of decision charts that helps determine the best status code for each situation. See the following for 4xx status codes:

HTTP is an extensible protocol and 422 is registered in IANA, which makes it a standard status code. So nothing stops you from using 422 in your application. And since June 2022, 422 is defined in the RFC 9110, which is the document that currently defines the semantics of the HTTP protocol:

15.5.21. 422 Unprocessable Content The 422 (Unprocessable Content) status code indicates that the server understands the content type of the request content (hence a 415 (Unsupported Media Type) status code is inappropriate), and the syntax of the request content is correct, but it was unable to process the contained instructions. For example, this status code can be sent if an XML request content contains well-formed (i.e., syntactically correct), but semantically erroneous XML instructions.