BUG: inconsistant MultiIndex behavior in .loc

[auderson]

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Reproducible Example

import pandas as pd
import numpy as np

df = pd.DataFrame(index=pd.MultiIndex.from_product([list('ab'), list('cd'), list('e')]), columns=['Val'])

print(df.loc[np.s_[:,'c',:]])
print(df.loc[np.s_[:,'c',:], "Val"])
print(df.loc[np.s_[:,'c',:], :])

Issue Description

When only selecting the index, the second level is dropped. But if I add a selector to the columns, the second level is kept.

Expected Behavior

I'm not sure if this is intended behavior, but it's inconsistant. FYI, I have a relevent report earlier: #43599

Installed Versions

INSTALLED VERSIONS

commit : a671b5a
python : 3.10.6.final.0
python-bits : 64
OS : Linux
OS-release : 5.15.0-89-generic
Version : #99-Ubuntu SMP Mon Oct 30 20:42:41 UTC 2023
machine : x86_64
processor : x86_64
byteorder : little
LC_ALL : None
LANG : en_US.UTF-8
LOCALE : en_US.UTF-8

pandas : 2.1.4
numpy : 1.26.2
pytz : 2023.3.post1
dateutil : 2.8.2
setuptools : 65.5.1
pip : 23.3.2
Cython : None
pytest : None
hypothesis : None
sphinx : None
blosc : None
feather : None
xlsxwriter : None
lxml.etree : None
html5lib : None
pymysql : None
psycopg2 : None
jinja2 : None
IPython : None
pandas_datareader : None
bs4 : None
bottleneck : None
dataframe-api-compat: None
fastparquet : None
fsspec : None
gcsfs : None
matplotlib : None
numba : None
numexpr : None
odfpy : None
openpyxl : None
pandas_gbq : None
pyarrow : None
pyreadstat : None
pyxlsb : None
s3fs : None
scipy : None
sqlalchemy : None
tables : None
tabulate : None
xarray : None
xlrd : None
zstandard : None
tzdata : 2023.3
qtpy : None
pyqt5 : None

[auderson]

Note in the second example, the column "Val" is also dropped. So I think maybe the "c" in the index should also be dropped.

[rhshadrach]

Thanks for the report. This appears to be a duplicate of #18631. Closing for now - if you think I've missed something, comment here and we can reopen.