YARD handles inheritance very strangely. Enabling --embed-mixins changes what is shown as included/extended, but with or without, the output is incorrect.

mixin_test.rb

# frozen_string_literal: true

# Test mixin
#
# {.mixin_class_attribute} and {.mixin_class_method} _never_ end up accessible
# in the inheriting class.
module TestMixin
  class << self
    attr_reader :mixin_class_attribute
    def mixin_class_method; end
  end
  attr_reader :mixin_instance_attribute
  def mixin_instance_method; end
end

# Includes the mixin
#
# == Actual Results
# === Class methods/attributes
# (none)
#
# === Instance methods/attributes
# * {TestMixin#mixin_instance_attribute}
# * {TestMixin#mixin_instance_method}
#
# == With --embed-mixins
# === Class methods/attributes
# * {TestMixin.mixin_class_attribute} (bad)
#
# === Instance methods/attributes
# * {TestMixin#mixin_instance_attribute}
# * {TestMixin#mixin_instance_method}
#
# == Without --embed-mixins
# === Class methods/attributes
# * {TestMixin.mixin_class_method} (bad)
#
# === Instance methods/attributes
# * {TestMixin#mixin_instance_attribute}
# * {TestMixin#mixin_instance_method}
class IncludeTestClass
  include TestMixin
end

# Extends the mixin
#
# == Actual Results
# === Class methods/attributes
# * {TestMixin#mixin_instance_attribute}
# * {TestMixin#mixin_instance_method}
#
# === Instance methods/attributes
# (none)
#
# == With --embed-mixins
# === Class methods/attributes
# * {TestMixin.mixin_class_attribute} (bad)
# * {TestMixin#mixin_instance_method}
#
# === Instance methods/attributes
# * {TestMixin#mixin_instance_attribute} (bad, should be class attribute)
#
# == Without --embed-mixins
# === Class methods/attributes
# * {TestMixin.mixin_class_method} (bad)
# * {TestMixin#mixin_instance_method}
#
# === Instance methods/attributes
# * {TestMixin#mixin_instance_attribute} (bad, should be class attribute)
class ExtendTestClass
  extend TestMixin
end