Describe the bug
Incorrect results

Steps/Code to reproduce bug

%%writefile t.py
import pandas as pd
df = pd.DataFrame({'a': [1, 2, None, 3], 'b': ['x']*4})
print(df.groupby('b')['a'].cumprod())

Then run

!python t.py

and

!python -m cudf.pandas t.py

Expected behavior

0    1.0
1    2.0
2    NaN
3    6.0
Name: a, dtype: float64

got

0    1.0
1    2.0
2    NaN
3    NaN
Name: a, dtype: float64

Environment overview (please complete the following information)

• Environment location: [Bare-metal, Docker, Cloud(specify cloud provider)]
• Method of cuDF install: [conda, Docker, or from source]
  • If method of install is [Docker], provide docker pull & docker run commands used

cudf version: '25.02.00a360'

Environment details
Please run and paste the output of the cudf/print_env.sh script here, to gather any other relevant environment details

Additional context
Add any other context about the problem here.

spotted in Narwhals