null and {$exists: false} queries inside arrays match less than I'd expect

If I understand properly, the general rule for doing a query

{'a.x': QUERY}

where 'a' is an array is that it should match if there is any element of the 'a' array where

{x: QUERY}

matches.

But Mongo 2.2.1 does this:

> db.x.remove({}); db.x.insert({a: [

{x: 1}

, {}]})
> db.x.findOne({'a.x': {$exists: false}})
null
> db.x.findOne(

{'a.x': null}

)
null

That surprises me: I'd expect both of these to match, because there is an element of the 'a' array (the {}) such that both {x: {$exists: false}} and

{x: null}

matches.

I don't see any relevant tests in the jstests directory asserting that the current behavior is expected. Is this behavior intentional or a bug?