TomekR

Seems that the root cause is not WP 6.3: I downgraded the site and the problem exists.
Reinstall of the Hestia hasn’t helped either.
Hitting the wall now, no clue what else could I do …

Hi Deepak,
Not answering directly to your question, but…

I don’t expect the plugin to respond in xml format, as even by name it is json :-). Also, this format is typical for REST services.
So instead, you should deserizalize it on your side and serialize to xml, if you need xml.

For all others interested:
I’ve achieved it by modifying introspector class, set_posts_query function, where I’ve just added this line:

$query['post_status'] = array( 'publish', 'future' );
before last if.

This is quite primitive way and probably will not work well in case of other post_status values (authorization consideration if status is draft) but works fine for planned posts.

btw
I don’t know php and maybe that’s why I don’t understand, how it works, that in the mentioned function there are lines of sort:
some_var = $json_api->query->post_type
i.e. post_type field is referenced, but I couldn’t find any definition/declaration of post_type field in query object, which is JSON_API_Query type, as far I understand.