Grails 3.1.1 - command object data binding does not play well with UrlMappings params

I have an issue regarding data binding from JSON body to command object. I can provide sample app, but for now here are relevant snippets

```
"/api/someEndpoint/$id?"(controller: 'book', action: 'index', method: 'POST')
```

```
class BookController {
def index(SomeCommandObject cmd) {
render ([paramsAre: params, commandIs: cmd] as JSON)
}
}

class SomeCommandObject {
Long id
String someString
Integer someInteger
}
```

When POST-ing empty request to :8080/api/someEndpoint/1 the 'id' field of a command object is bound correctly, however including JSON, like this (using httpie)

```
http POST :8080/api/someEndpoint/1 someString=abc
```

causes 'someString' to bind but 'id' is null. I would expect these two sources of data (body and params coming from UrlMappings) to be merged. This issue seems relevant https://jira.grails.org/browse/GRAILS-11263

Reproduce the issue with the shown UrlMappings entry, BookController.index, SomeCommandObject, and the HTTPie POST examples. Read the command-object data-binding path and the referenced GRAILS-11263 context; done means a JSON body binds someString while the UrlMappings id remains available as 1.

Written by the indexing model from the issue text.