`:undoc-members:` option doesn't work for instance attributes

Describe the bug

The :undoc-members: option does not include instance attributes without docstrings when used with the .. autoclass:: directive.

How to Reproduce

example.py

class Example:
    """A simple example."""

    def __init__(self, one_arg: str = "value"):

        self.instance_var1 = 123
        self.instance_var2 = one_arg

example.rst

example
*******

.. autoclass:: example.Example
    :members:
    :undoc-members:

conf.py

import os
import sys
sys.path.insert(0, os.path.abspath(os.path.join('..', '..')))

html_theme = 'sphinx_rtd_theme'
html_static_path = ['_static']

extensions = [
    'sphinx.ext.autodoc',
    'sphinx_rtd_theme'
]

autoclass_content = 'class'
autodoc_class_signature = "mixed"
autodoc_typehints = 'signature'
autodoc_preserve_defaults = True
add_module_names = False
add_function_parentheses = True
autodoc_typehints_description_target="all"

Expected behavior

Instance attributes without docstrings should be included by autodoc in the documentation if the :undoc-members: option is specified.

Your project

Personal project

Screenshots

OS

Windows 10 Pro

Python version

3.9.0

Sphinx version

4.4.0

Sphinx extensions

autodoc

Extra tools

None

Additional context

No response

Start with the provided example.py, example.rst, and conf.py reproduction, focusing on the autoclass and undoc-members behavior described in the issue. Confirm that the instance attributes are omitted, then ensure the completed behavior includes undocumented instance attributes in the generated documentation when :undoc-members: is enabled.