BUG: loc returning wrong elements for non-monotonic DatetimeIndex
- [x] closes #33146
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

That one was tricky. When only one existing key was given or both string parts exist, ``Index.slice_indexer`` is not raising KeyError but selecting a lof of erroneous values