BUG: invalid result of reindex on columns after unstack with Period data

[a-rvn]

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Reproducible Example

import pandas as pd

series1 = pd.DataFrame(
   [(0, "s2", pd.Period(2022)), (0, "s1", pd.Period(2021))],
   columns=["A", "B", "C"]
).set_index(["A", "B"])["C"]

series2 = series1.astype(str)

print(series1.unstack("B").reindex(["s2"], axis=1))
print(series2.unstack("B").reindex(["s2"], axis=1))

Issue Description

The example code prints

B  s2
A      
0  2021

B  s2
A  
0  2022

The result with pd.Period data is the incorrect 2021, but with str data it's the correct 2022.

This only occurs with certain index values. When replacing "s1" with "s3", the effect disappears and the result is 2022 in both cases.

Expected Behavior

Expect the result for both pd.Period and str data to be 2022:

B  s2
A  
0  2022

B  s2
A  
0  2022

(actually observed with older Pandas 2.0.3)

Installed Versions

INSTALLED VERSIONS

commit : 0691c5c
python : 3.11.10
python-bits : 64
OS : Linux
OS-release : 6.2.16
Version : #1-NixOS SMP PREEMPT_DYNAMIC Tue Jan 1 00:00:00 UTC 1980
machine : x86_64
processor :
byteorder : little
LC_ALL : None
LANG : en_US.UTF-8
LOCALE : en_US.UTF-8
pandas : 2.2.3
numpy : 2.2.3
pytz : 2025.1
dateutil : 2.9.0.post0
pip : 24.0
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 : 2025.1
qtpy : None
pyqt5 : None