REGR: SeriesGrouper using Timestamp index
- [x] closes https://github.com/pandas-dev/pandas/issues/42390
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [x] whatsnew entry

Not sure what the defining characteristic of the failing cases is so there's a blank in the whatsnew note.
@meeseeksdev backport 1.3.x Something went wrong ... Please have  a look at my logs.