clientMutationId field no longer working in my Ruby GraphQL schema

Question

Previously the mutation was defined like this:

class Mutations::SomeMutation < Mutations::BaseMutation

  null false

  argument :some_argument, String, required: true

  field :some_field, String, null: false

  def resolve(some_argument:)
    ...
  end
end

And the following mutation would return the clientMutationId when specified.

mutation {
  someMutation(
    input: {
      someArgument: "string"
      clientMutationId: "123"
    }
  ) {
    someField
    clientMutationId
  }
}

{
  "data": {
    "someMutation": {
      "someField": "hello",
      "clientMutationId": "123"
    }
  }
}

However after adding specific classes for the arguments and return type it no longer works without specifically adding the clientMutationId field to the return type:

class Mutations::SomeMutation < Mutations::BaseMutation

  null false

  class SomeInput < Types::BaseInputObject
    argument :some_argument, String, required: true
  end
  
  input_object_class SomeInput

  class SuccessResult < Types::BaseObject
    field :some_field, String, null: false
    field :client_mutation_id, String, null: true
  end

  class SomeMutationResult < Types::BaseUnion
    possible_types SuccessResult, ErrorType

    def self.resolve_type(object, _context)
      object[:some_field].valid? ? SuccessResult : ErrorType
    end
  end

  type SomeMutationResult

  def resolve(some_argument:)
    ...
  end
end

I'm guessing because the return type does not exist inside the class that inherits from BaseMutation it loses this functionality.

Here is what the base class looks like:

module Mutations
  class BaseMutation < GraphQL::Schema::RelayClassicMutation
    argument_class Types::BaseArgument
    field_class Types::BaseField
    input_object_class Types::BaseInputObject
    object_class Types::BaseObject
  end
end

All of these classes are just inheriting from the standard GraphQL classes from the Gem.