Steps to reproduce

I'll provide more details if you don't get it. Or create a pull request when I can.

Expected behavior

app.as_json(current_resource_owner: app.owner)['id'].is_a?(String) == true

Actual behavior

app.as_json(current_resource_owner: app.owner)['id'].is_a?(String) != true
app.as_json(current_resource_owner: app.owner)['id'].is_a?(BSON::ObjectId) == true

System configuration

Ruby version:
2.7.3

Gemfile.lock
doorkeeper (5.5.2)
doorkeeper-mongodb (5.2.2)
mongo (2.14.0)
mongoid (6.4.8)
rails (5.2.6)