BUG: align not adding levels only on one side if intersection is equal
- [x] closes #45224
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit) for how to run them
- [x] whatsnew entry

This is now consistent with the case when both sides do not have the same intersection of indexes
thanks @phofl 