There is currently an [override](https://gitlab.com/gitlab-org/gitlab/-/blob/7438977bb73a48a38aa6d318d06aba9cd6573f3b/app/models/concerns/deprecated_assignee.rb#L45) for `assignees` association in `app/models/concerns/deprecated_assignee.rb` This is an override for a has_many through: association [here](https://gitlab.com/gitlab-org/gitlab/-/blob/7438977bb73a48a38aa6d318d06aba9cd6573f3b/app/models/merge_request.rb#L104). Overriding `through:` associations can turn out to be problematic, as described in https://docs.gitlab.com/ee/development/gotchas.html#do-not-override-has_many-through-or-has_one-through-associations and needs to be removed. References: https://gitlab.com/gitlab-org/gitlab/-/issues/424536