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BUG: Inconsistent behavior of Groupby with None values with filter #62501

@chiiyeh

Description

@chiiyeh

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Reproducible Example

import pandas as pd

df = pd.DataFrame(data=[
    {"a": 1, "b": None},
    {"a": 2, "b": None},
])
df.groupby("b", dropna=False).filter(lambda x: True)

Issue Description

Expected to return the full df, but it returns an empty df instead. Also inconsistent behavior when the first row, b col is changed to a number e.g. 10 (The row with None value is not returned) vs when it is changed to a string e.g. "a" (The row with None value is returned)

Expected Behavior

Expect it to return the full dataframe but it returns an empty df instead.

Installed Versions

Details INSTALLED VERSIONS ------------------ commit : 4665c10 python : 3.10.12 python-bits : 64 OS : Linux OS-release : 5.4.0-1154-azure-fips Version : #161+fips1-Ubuntu SMP Fri Jun 27 04:05:20 UTC 2025 machine : x86_64 processor : x86_64 byteorder : little LC_ALL : None LANG : C.UTF-8 LOCALE : en_US.UTF-8

pandas : 2.3.2
numpy : 1.24.4
pytz : 2022.1
dateutil : 2.8.2
pip : 22.2.2
Cython : 0.29.32
sphinx : None
IPython : 8.10.0
adbc-driver-postgresql: None
adbc-driver-sqlite : None
bs4 : None
blosc : None
bottleneck : None
dataframe-api-compat : None
fastparquet : None
fsspec : None
html5lib : None
hypothesis : None
gcsfs : None
jinja2 : 3.1.6
lxml.etree : None
matplotlib : None
numba : None
numexpr : None
odfpy : None
openpyxl : None
pandas_gbq : None
psycopg2 : None
pymysql : None
pyarrow : None
pyreadstat : None
pytest : None
python-calamine : None
pyxlsb : None
s3fs : None
scipy : None
sqlalchemy : None
tables : None
tabulate : None
xarray : None
xlrd : None
xlsxwriter : None
zstandard : None
tzdata : 2025.2
qtpy : None
pyqt5 : None
/databricks/python/lib/python3.10/site-packages/_distutils_hack/init.py:33: UserWarning: Setuptools is replacing distutils.
warnings.warn("Setuptools is replacing distutils.")

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BugFilterse.g. head, tail, nthGroupbyMissing-datanp.nan, pd.NaT, pd.NA, dropna, isnull, interpolate

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