`--last-failed` does not (always?) see tests which failed in a subtest

When running a unittest-based test suite which makes use of subtests, failed subtests are correctly reported at the end of a run (possibly a bit too much as it's hitting #13905 / #13986), however when re-running tests using --lf they seem to skipped.

Originally observed on pytest 9.0.2 / uv python 3.12, reproduced on 9.0.3 / uv python 3.14, running on linux mint 22.3

$ uv run pytest
==== test session starts ===
platform linux -- Python 3.14.2, pytest-9.0.3, pluggy-1.6.0
rootdir: /tmp/testdir
configfile: pytest.ini
collected 1 item                                                                                                                                                                                                                             

test_qux.py .                                                                                                                                                                                                                        [100%]

=== FAILURES ===
__________________________________________________________________________________________________________ FooTest.testX (index=1) ___________________________________________________________________________________________________________

self = <test_qux.FooTest testMethod=testX>

    def testX(self):
        for i in range(5):
            with self.subTest(index=i):
>               self.assertEqual(i % 2, 0)
E               AssertionError: 1 != 0

test_qux.py:9: AssertionError
__________________________________________________________________________________________________________ FooTest.testX (index=3) ___________________________________________________________________________________________________________

self = <test_qux.FooTest testMethod=testX>

    def testX(self):
        for i in range(5):
            with self.subTest(index=i):
>               self.assertEqual(i % 2, 0)
E               AssertionError: 1 != 0

test_qux.py:9: AssertionError
======================================================================================================= short test summary info ===
SUBFAILED(index=1) test_qux.py::FooTest::testX - AssertionError: 1 != 0
SUBFAILED(index=3) test_qux.py::FooTest::testX - AssertionError: 1 != 0
=== 2 failed, 1 passed in 0.04s ===
$ uv run pytest --cache-show 
=== test session starts ===
platform linux -- Python 3.14.2, pytest-9.0.3, pluggy-1.6.0
rootdir: /tmp/testdir
configfile: pytest.ini
cachedir: /tmp/testdir/.pytest_cache
--- cache values for '*' ---
cache/nodeids contains:
  ['test_qux.py::FooTest::testX']

=== no tests ran in 0.00s ===
$ uv run pytest --lf --lfnf=none
=== test session starts ===
platform linux -- Python 3.14.2, pytest-9.0.3, pluggy-1.6.0
rootdir: /tmp/testdir
configfile: pytest.ini
collected 1 item / 1 deselected / 0 selected                                                                                                                                                                                                 
run-last-failure: no previously failed tests, deselecting all items.

=== 1 deselected in 0.00s ===

# test_qux.py
import unittest


class FooTest(unittest.TestCase):
    def testX(self):
        for i in range(5):
            with self.subTest(index=i):
                self.assertEqual(i % 2, 0)

# pyproject.toml (mostly so uv knows to fetch pytest)
[project]
name = "testdir"
version = "0.1.0"
description = "Add your description here"
readme = "README.md"
requires-python = ">=3.14"
dependencies = []

[dependency-groups]
dev = [
    "pytest>=9.0.3",
]

And an empty pytest.ini to ensure pytest gets the correct rootdir (not sure it's necessary with the pyproject.toml but...)

Start with the reproducer in test_qux.py and run the shown pytest, --cache-show, and --lf commands using the pyproject.toml setup. Trace how unittest subTest failures are reported and stored for --last-failed; done means the failed subtests are recorded and selected on the next run without breaking ordinary failure handling.