API: transform behaves differently with 'ffill' on DataFrameGroupBy and SeriesGroupBy
- [ ] closes #24211
- [ ] tests added / passed
- [ ] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [ ] whatsnew entry

Thanks @ftrihardjo - could you make the title the same as the issue please, and put the issue number in the pull request body (near `closes #`) ? thanks @ftrihardjo 