Pandas BUG: mask in test_mask_stringdtype would always return the same result regardless of cond

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• [x] I have confirmed this bug exists on the latest version of pandas.

• [x] I have confirmed this bug exists on the main branch of pandas.

Reproducible Example

import pandas as pd

# test_mask_stringdtype
obj = pd.DataFrame(
    {"A": ["foo", "bar", "baz", pd.NA]},
    index=["id1", "id2", "id3", "id4"],
    dtype=pd.StringDtype(),
)
filtered_obj = pd.DataFrame(
    {"A": ["this", "that"]}, index=["id2", "id3"], dtype=pd.StringDtype()
)
expected = pd.DataFrame(
    {"A": [pd.NA, "this", "that", pd.NA]},
    index=["id1", "id2", "id3", "id4"],
    dtype=pd.StringDtype(),
)

filter_ser = pd.Series([False, True, True, False])
obj.mask(filter_ser, filtered_obj)
#         A
# id1  <NA>
# id2  this
# id3  that
# id4  <NA>

filter_ser = pd.Series([True, False, False, True])
obj.mask(filter_ser, filtered_obj)
#         A
# id1  <NA>
# id2  this
# id3  that
# id4  <NA>

filter_ser = pd.Series([False, False, False, False])
obj.mask(filter_ser, filtered_obj)
#         A
# id1  <NA>
# id2  this
# id3  that
# id4  <NA>

filter_ser = pd.Series([True, True, True, True])
obj.mask(filter_ser, filtered_obj)
#         A
# id1  <NA>
# id2  this
# id3  that
# id4  <NA>

Issue Description

Found during #60772 . I suppose the purpose of this test is to check if mask works as expected with pd.StringDtype() (See #40824 ), but the test seems to return the same result regardless of cond since it fails to align in _where.

If we want to check if mask replaces with other only where cond is True and let cond propagate where cond is False, I think filter_ser should have index so that mask can recognize the corresponding other value.

Expected Behavior

filter_ser = pd.Series([False, True, True, False], index=["id1", "id2", "id3", "id4"])
obj.mask(filter_ser, filtered_obj)
#         A
# id1   foo
# id2  this
# id3  that
# id4  <NA>

Installed Versions

commit : c888af6d0bb674932007623c0867e1fbd4bdc2c6 python : 3.12.7 python-bits : 64 OS : Windows OS-release : 11 Version : 10.0.26100 machine : AMD64 processor : AMD64 Family 25 Model 80 Stepping 0, AuthenticAMD byteorder : little LC_ALL : None LANG : None LOCALE : Korean_Korea.949 pandas : 2.3.1 numpy : 2.3.1 pytz : 2025.2 dateutil : 2.9.0.post0 pip : 24.2 Cython : None sphinx : None IPython : None 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 : None 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

Comment From: arthurlw

Confirmed on main. PRs and investigations are welcome

Thanks for raising this!