Hiding WP_Query params under the filter key (instead of allowing them to be top-level params) was one of our biggest complaints from users of v1 of our REST API. In Beta 15 the WP_Query vars in the Posts Controller were moved under the filter param, inspired by our v1 architecture. 🙍

Now we have an inconsistent mess. See #2192 and #2066 (and likely many other filed or unfiled tickets).

I suggest walking back this change prior to any WP Core merge.