pandas: BUG: groupby transform doesn't respect Series index anymore

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Reproducible Example

df = pd.DataFrame({
    'A': ['foo', 'bar', 'foo', 'bar', 'bar', 'foo', 'foo'],
    'C': [2.1, 1.9, 3.6, 4.0, 1.9, 7.8, 2.8]
}).convert_dtypes()

print(df)
     A    C
0  foo  2.1
1  bar  1.9
2  foo  3.6
3  bar  4.0
4  bar  1.9
5  foo  7.8
6  foo  2.8

# sort C within groups defined by A
df.groupby('A')['C'].transform(pd.Series.sort_values)
     C
0  2.1
1  1.9
2  3.6
3  4.0
4  1.9
5  7.8
6  2.8

Issue Description

Suppose I have the following DataFrame

df = pd.DataFrame({
    'A': ['foo', 'bar', 'foo', 'bar', 'bar', 'foo', 'foo'],
    'C': [2.1, 1.9, 3.6, 4.0, 1.9, 7.8, 2.8]
}).convert_dtypes()

print(df)
     A    C
0  foo  2.1
1  bar  1.9
2  foo  3.6
3  bar  4.0
4  bar  1.9
5  foo  7.8
6  foo  2.8

I used to be able to sort C’s values within the groups defined by A like this (proof):

df.groupby('A')['C'].transform(pd.Series.sort_values)
     C
0  2.1
1  1.9
2  2.8
3  1.9
4  4.0
5  3.6
6  7.8

(This was sometime around Pandas 1.0.0)

However pandas 1.4.0 produces the following

df.groupby('A')['C'].transform(pd.Series.sort_values)
     C
0  2.1
1  1.9
2  3.6
3  4.0
4  1.9
5  7.8
6  2.8

It’s as if the sort_values() function isn’t even being applied.

Note that df.groupby('A')[['C']].transform(pd.Series.sort_values) works properly.

df.groupby('A')[['C']].transform(pd.Series.sort_values)
     C
0  2.1
1  1.9
2  2.8
3  1.9
4  4.0
5  3.6
6  7.8

Expected Behavior

I would expect df.groupby('A')['C'].transform(pd.Series.sort_values) to actually sort C’s values within the groups defined by A (as it once did). My expected output in this example would be a Series like this.

     C
0  2.1
1  1.9
2  2.8
3  1.9
4  4.0
5  3.6
6  7.8

Installed Versions

INSTALLED VERSIONS

commit : bb1f651536508cdfef8550f93ace7849b00046ee python : 3.10.1.final.0 python-bits : 64 OS : Darwin OS-release : 21.2.0 Version : Darwin Kernel Version 21.2.0: Sun Nov 28 20:28:54 PST 2021; root:xnu-8019.61.5~1/RELEASE_X86_64 machine : x86_64 processor : i386 byteorder : little LC_ALL : None LANG : None LOCALE : en_US.UTF-8 pandas : 1.4.0 numpy : 1.22.1 pytz : 2021.3 dateutil : 2.8.2 pip : 21.1.2 setuptools : 57.0.0 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 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 xlwt : None zstandard : None