BUG: Series.loc[dtstr] type depends on monotonicity
- [x] closes #24892
- [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
- [ ] whatsnew entry

xref #42258 which makes the same change for PeriodIndex._can_partial_date_slice but its behavior-neutral there.

cc @mroeschke 