id,node_id,number,title,user,state,locked,assignee,milestone,comments,created_at,updated_at,closed_at,author_association,active_lock_reason,draft,pull_request,body,reactions,performed_via_github_app,state_reason,repo,type
1238783899,I_kwDOAMm_X85J1leb,6616,flox breaks multiindex groupby,2448579,closed,0,,,0,2022-05-17T15:05:00Z,2022-05-17T16:11:18Z,2022-05-17T16:11:18Z,MEMBER,,,,"### What happened?

From @malmans2 

``` python
import numpy as np

import xarray as xr

ds = xr.Dataset(
    dict(a=((""z"",), np.ones(10))),
    coords=dict(b=((""z""), np.arange(2).repeat(5)), c=((""z""), np.arange(5).repeat(2))),
).set_index(bc=[""b"", ""c""])
grouped = ds.groupby(""bc"")

with xr.set_options(use_flox=False):
    grouped.sum()  # OK

with xr.set_options(use_flox=True):
    grouped.sum()  # Error
```

### What did you expect to happen?

_No response_

### Minimal Complete Verifiable Example

_No response_

### MVCE confirmation

- [ ] Minimal example — the example is as focused as reasonably possible to demonstrate the underlying issue in xarray.
- [ ] Complete example — the example is self-contained, including all data and the text of any traceback.
- [ ] Verifiable example — the example copy & pastes into an IPython prompt or [Binder notebook](https://mybinder.org/v2/gh/pydata/xarray/main?urlpath=lab/tree/doc/examples/blank_template.ipynb), returning the result.
- [ ] New issue — a search of GitHub Issues suggests this is not a duplicate.

### Relevant log output

```Python
ctests/test_xarray.py:329: in test_multi_index_groupby_sum
    actual = xarray_reduce(ds, ""bc"", func=""sum"")
flox/xarray.py:374: in xarray_reduce
    actual[k] = v.expand_dims(missing_group_dims)
../xarray/xarray/core/dataset.py:1427: in __setitem__
    self.update({key: value})
../xarray/xarray/core/dataset.py:4432: in update
    merge_result = dataset_update_method(self, other)
../xarray/xarray/core/merge.py:1070: in dataset_update_method
    return merge_core(
../xarray/xarray/core/merge.py:722: in merge_core
    aligned = deep_align(
../xarray/xarray/core/alignment.py:824: in deep_align
    aligned = align(
../xarray/xarray/core/alignment.py:761: in align
    aligner.align()
../xarray/xarray/core/alignment.py:550: in align
    self.assert_unindexed_dim_sizes_equal()
../xarray/xarray/core/alignment.py:450: in assert_unindexed_dim_sizes_equal
    raise ValueError(
E   ValueError: cannot reindex or align along dimension 'bc' because of conflicting dimension sizes: {10, 6} (note: an index is found along that dimension with size=10)
______________________________ test_multi_index_groupby_sum[numpy] _______________________________
tests/test_xarray.py:329: in test_multi_index_groupby_sum
    actual = xarray_reduce(ds, ""bc"", func=""sum"")
flox/xarray.py:374: in xarray_reduce
    actual[k] = v.expand_dims(missing_group_dims)
../xarray/xarray/core/dataset.py:1427: in __setitem__
    self.update({key: value})
../xarray/xarray/core/dataset.py:4432: in update
    merge_result = dataset_update_method(self, other)
../xarray/xarray/core/merge.py:1070: in dataset_update_method
    return merge_core(
../xarray/xarray/core/merge.py:722: in merge_core
    aligned = deep_align(
../xarray/xarray/core/alignment.py:824: in deep_align
    aligned = align(
../xarray/xarray/core/alignment.py:761: in align
    aligner.align()
../xarray/xarray/core/alignment.py:550: in align
    self.assert_unindexed_dim_sizes_equal()
../xarray/xarray/core/alignment.py:450: in assert_unindexed_dim_sizes_equal
    raise ValueError(
E   ValueError: cannot reindex or align along dimension 'bc' because of conflicting dimension sizes: {10, 6} (note: an index is found along that dimension with size=10)

```
```


### Anything else we need to know?

_No response_

### Environment

<details>



</details>
","{""url"": ""https://api.github.com/repos/pydata/xarray/issues/6616/reactions"", ""total_count"": 1, ""+1"": 1, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,completed,13221727,issue