dmypy not reporting an issue that mypy finds

Bug Report

packaging on  enriched-metadata [!⇡] via 🐍 v3.11.4 (.venv) 
❯ PYTHONPATH=/mnt/c/Users/brcan/.vscode-insiders/extensions/ms-python.mypy-type-checker-2023.1.11921011/bundled/libs /hom
e/brettcannon/Repositories/pypa/packaging/.venv/bin/python -m mypy --no-color-output --no-error-summary --show-absolute-p
ath --show-column-numbers --show-error-code --no-pretty --follow-imports=normal --show-error-end /home/brettcannon/Reposi
tories/pypa/packaging/src/packaging/metadata.py
/home/brettcannon/Repositories/pypa/packaging/src/packaging/metadata.py:303:18:303:24: error: Item "None" of "list[Any] | None" has no attribute "__iter__" (not iterable)  [union-attr]
/home/brettcannon/Repositories/pypa/packaging/src/packaging/metadata.py:604:5:624:86: error: Function is missing a type annotation for one or more arguments  [no-untyped-def]
⏎                                                                                                                   

packaging on  enriched-metadata [+⇡] via 🐍 v3.11.4 (.venv) 
❯ PYTHONPATH=/mnt/c/Users/brcan/.vscode-insiders/extensions/ms-python.mypy-type-checker-2023.1.11921011/bundled/libs
 /home/brettcannon/Repositories/pypa/packaging/.venv/bin/python -m mypy.dmypy --status-file /tmp/.vscode.dmypy_statu
s/status-55f8413f-94ce-437f-9911-110647253f9d.json run -- --no-color-output --no-error-summary --show-absolute-path 
--show-column-numbers --show-error-code --no-pretty --follow-imports=normal --show-error-end /home/brettcannon/Repos
itories/pypa/packaging/src/packaging/metadata.py

packaging on  enriched-metadata [+⇡] via 🐍 v3.11.4 (.venv)

To Reproduce

See above.

Expected Behavior

Issues reported by mypy to also be reported by dmypy.

Actual Behavior

dmypy not printing any issues.

Your Environment

• Mypy version used: mypy 1.4.1 (compiled: no)
• Mypy command-line flags:
• Mypy configuration options from mypy.ini (and other config files): https://github.com/pypa/packaging/blob/1277904652c497ec84560a830a2e01d434358818/pyproject.toml#L51-L54
• Python version used: 3.11.4

Start by reproducing the reported commands against packaging/src/packaging/metadata.py, comparing the regular mypy and dmypy outputs with mypy 1.4.1 and the listed flags. Trace the dmypy command path and add a regression test for the discrepancy. Done means dmypy reports the same issues as mypy for this case.