Update from mongoid 6.4.0 to 8.1: has_many broken?

[DcsMarionDickten]

I'll just post the broken code. It worked in Rails 5.2 with Mongoid 6.4 but doesn't in Rails 6.2 with Mongoid 8.1.

module Answers

  class Participant
    include Mongoid::Document
    include Mongoid::Timestamps
    include Mongoid::Attributes::Dynamic

    # ... field definitions

    has_many :participations, class_name: '::Answers::Participation'

    after_destroy :destroy_participations

   # more code

Participations snippet:

module Answers

  class Participation
    include Mongoid::Document
    include Mongoid::Timestamps
    include Mongoid::Attributes::Dynamic

    belongs_to :participant, class_name: '::Answers::Participant'
    # ... more fields, then more code

The has_many has broken when upgrading to Mongoid 7.4 and is still broken in 8.1. I can still get the participations by asking

def participations
     Answers::Participation.where(participant_id: self.id)
end

so it's not too bad although I would like to fix it properly.
Can anybody point me to what is wrong?