BUG: Strings in Excel number formats do not preserve case

[alexklapheke]

Pandas version checks

• I have checked that this issue has not already been reported.

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

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

Reproducible Example

import pandas as pd

data = pd.DataFrame({"col": [1e6, 2e6, 3e6]})
css = 'number-format: #,,"M";'  # format 1000000 as '1M'
data.style.apply(lambda series: [css] * len(series)).to_excel("test.xlsx")

Issue Description

When exporting an Excel file with a custom number format, any custom strings (in double quotes) are lower-cased. This happens with both engine="openpyxl" and engine="xlsxwriter". Here, e.g., the string literal "M" is converted to "m":

Expected Behavior

String literals in custom number formats should be unchanged.

Installed Versions

INSTALLED VERSIONS
------------------
commit                : 415830fc8f9c77a8031f84082bef5cc441c2d13f
python                : 3.13.9
python-bits           : 64
OS                    : Linux
OS-release            : 6.6.87.2-microsoft-standard-WSL2
Version               : #1 SMP PREEMPT_DYNAMIC Thu Jun  5 18:30:46 UTC 2025
machine               : x86_64
processor             : x86_64
byteorder             : little
LC_ALL                : None
LANG                  : en_US.UTF-8
LOCALE                : en_US.UTF-8

pandas                : 3.0.0.dev0+2709.g415830fc8f
numpy                 : 2.4.0.dev0+git20251111.bf56676
dateutil              : 2.9.0.post0
pip                   : 25.2
Cython                : None
sphinx                : None
IPython               : None
adbc-driver-postgresql: None
adbc-driver-sqlite    : None
bs4                   : None
bottleneck            : 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              : 3.1.5
psycopg2              : None
pymysql               : None
pyarrow               : None
pyiceberg             : None
pyreadstat            : None
pytest                : None
python-calamine       : None
pytz                  : None
pyxlsb                : 1.0.10
s3fs                  : None
scipy                 : None
sqlalchemy            : None
tables                : None
tabulate              : None
xarray                : None
xlrd                  : 2.0.2
xlsxwriter            : 3.2.9
zstandard             : None
qtpy                  : None
pyqt5                 : None

[parthkandharkar]

Hi,
This is an interesting issue. Can you please assign it to me?