It looks like some cases in jstests/core/cover_null_queries.js (once the PR for SERVER-70436 is merged) return incorrect output on CQF passthroughs.

Specifically, the command coll.find({"a.b": null}, {_id: 1}) matches the following documents, which are not matched in the classic engine:
{_id: 14, a: [1, 2, 3]},
{_id: 16, a: [null]},
 
Note that the only index is

.