Heading in only directive gets displaced

[jdoubleu]

Describe the bug

When including a heading inside the .. only:: directive, it is somehow reordered.

Expected output:

1. H's
   1. Subheading
      1. <text>
   2. References
      1. <citations>
      2. <text>

Actual output:

As you can see, the References header is shown after the citations and even the paragraph.

When trying to reproduce this issue, I noticed, that it only works if there is another heading of the same level before the .. only:: directive.

How to Reproduce

index.rst:

####################################
Sphinx Bug Heading In Only Directive
####################################
   
Reproduction of the bug.

H's
===

Subheading
----------

This is a subheading.

.. only:: html

   References
   ----------

   This paragraph is only displayed when generating the HTML output.

.. [CIT] Example citation

This paragraph should be shown *after* the "References" header [CIT]_.

P's
===

.. only:: html

   On the other hand, whenever the :code:`only` directive does **not** include a heading, the order is as expected.

This paragraph is shown after the above, at least for HTML output.

Environment Information

Platform:              win32; (Windows-10-10.0.22631-SP0)
Python version:        3.11.6 (tags/v3.11.6:8b6ee5b, Oct  2 2023, 14:57:12) [MSC v.1935 64 bit (AMD64)])
Python implementation: CPython
Sphinx version:        8.0.2
Docutils version:      0.21.2
Jinja2 version:        3.1.4
Pygments version:      2.18.0

Sphinx extensions

[]

Additional context

No response