BUG: pandas groupby returns different results when one and two groups

[ibigquant]

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

# df = pd.DataFrame({"a": [1, 1, 1, 1], "b": [1, 2, 3, 4]})
df = pd.DataFrame({"a": [1, 2, 2, 1], "b": [1, 2, 3, 4]})

def foo(xf):
    def bar(x):
        # here just some sample code ..
        return x['b'] + 1

    return xf.apply(bar, axis=1)

result_series = df.groupby("a", group_keys=False).apply(foo)

result_series

Issue Description

it returns dataframe when there's only one group

Expected Behavior

always returns series

Installed Versions

INSTALLED VERSIONS

commit : bdc79c1
python : 3.11.8.final.0
python-bits : 64
OS : Linux
OS-release : 5.14.0-162.23.1.el9_1.x86_64
Version : #1 SMP PREEMPT_DYNAMIC Tue Apr 11 19:09:37 UTC 2023
machine : x86_64
processor : x86_64
byteorder : little
LC_ALL : zh_CN.UTF-8
LANG : zh_CN.UTF-8
LOCALE : zh_CN.UTF-8
pandas : 2.2.1
numpy : 1.26.4
pytz : 2024.1
dateutil : 2.9.0.post0
setuptools : 65.5.0
pip : 24.0
Cython : 3.0.9
pytest : 8.1.1
hypothesis : None
sphinx : None
blosc : None
feather : None
xlsxwriter : None
lxml.etree : 5.1.0
html5lib : None
pymysql : None
psycopg2 : None
jinja2 : 3.1.3
IPython : 8.22.2
pandas_datareader : 0.10.0
adbc-driver-postgresql: None
adbc-driver-sqlite : None
bs4 : 4.12.3
bottleneck : None
dataframe-api-compat : None
fastparquet : None
fsspec : None
gcsfs : None
matplotlib : 3.8.3
numba : 0.59.1
numexpr : None
odfpy : None
openpyxl : None
pandas_gbq : None
pyarrow : 13.0.0
pyreadstat : None
python-calamine : None
pyxlsb : None
s3fs : None
scipy : 1.12.0
sqlalchemy : 2.0.29
tables : None
tabulate : None
xarray : None
xlrd : 2.0.1
zstandard : None
tzdata : 2024.1
qtpy : None
pyqt5 : None

[shriyakalakata]

I have run the example above and it returned the expected behavior.

My output:

0    2
3    5
1    3
2    4
dtype: int64

Pandas version: 2.2.1

[rhshadrach]

Thanks for the report, closing as a duplicate of #54992