embeds_many mishandles positional projections (foo.$)

I have a `SingleSelectQuestion` model which `embeds_many :select_options`. In Mongoid 6.4.4, I can do this query to get me the first select option embedded within a `SingleSelectQuestion` that is active. Active is a Ruby method, defined as "status == :active".

Here's the query:

SingleSelectQuestion

  .where(

    id: question.id.to_s,

   "select_options._id" => question.select_options.first.id

  )

  .only("select_options.$")

  .first

  .select_options

  .detect(&:active?)

 There is only ever one "active" select option here. Due to how our database is constructed (the use of embeds many), I feel like this is the best way to do this query.

The thing I want to talk about today is that `only("select_options.$")` syntax.

When I run this code using the latest `mongodb/mongoid` master commit from today (currently b598df2dd78d), this query returns:

>> SingleSelectQuestion.where(id: question.id.to_s, "select_options._id" => question.select_options.first.id).only("select_options.$").first.select_options.detect(&:active?) Traceback (most recent call last): 3: from (irb):8 2: from (irb):8:in `detect' 1: from app/models/select_option.rb:74:in `active?' ActiveModel::MissingAttributeError (Missing attribute: 'status'.) 

I think the behaviour of `only` has regressed / changed here, as it appears the syntax of "select_options.$" is NOT including all the fields as it has done in the past.

Has something changed here? Is this a bug? Or is this an incorrect use of mongoid?