When updating a field containing an array of hashes using `update!`, the in-memory representation of the updated value differs depending on the `legacy_attributes` configuration.

Specifically:

• `legacy_attributes=true`: the updated value appears immediately in a normalized (string-keyed) form.
• `legacy_attributes=false`: symbol keys are retained in memory.

Is this an intended change?

Steps to Reproduce:

class Request
  include Mongoid::Document
  field :params, type: Array
end

In Rails console

Request.create!
req = Request.first

# Legacy attributes: false
req.update!(params: [{ date_from: "2025-05-01", date_to: "2025-05-31" }, 1, 100])
req.params
=> [{:date_from=>"2025-05-01", :date_to=>"2025-05-31"}, 1, 100] # Symbol key

# Legacy attributes: true
req.update!(params: [{ date_from: "2025-05-01", date_to: "2025-05-31" }, 1, 100])
req.params
=> [{"date_from"=>"2025-05-01", "date_to"=>"2025-05-31"}, 1, 100] # String key

Observed Behavior:

• 
  With legacy_attributes: false, the updated value in memory retains symbol keys and reflects the exact structure passed to update!.
• With legacy_attributes: true, the updated value in memory is immediately normalized to string keys — as though the document had been reloaded from the database. This suggests an implicit reload or internal refresh of attributes is occurring.

Expected Behavior:

• We expect that the in-memory representation of updated attributes remains consistent regardless of the legacy_attributes setting.
• We expect that values passed to update! are consistently normalized (mongoized) before being returned through accessors like `req.params`.

Environment:

• Mongoid version: 8.1.10

• MongoDB version: 7.0.11

• mongo-ruby-driver version: 2.21.0

• Ruby version: 3.3.6

• Rails version: 7.2.2.1