html_url,issue_url,id,node_id,user,created_at,updated_at,author_association,body,reactions,performed_via_github_app,issue
https://github.com/pydata/xarray/issues/6836#issuecomment-1498119367,https://api.github.com/repos/pydata/xarray/issues/6836,1498119367,IC_kwDOAMm_X85ZS3zH,2448579,2023-04-05T20:35:20Z,2023-04-05T20:35:20Z,MEMBER,"I think we could special-case extracting a multiindex level here:
https://github.com/pydata/xarray/blob/d4db16699f30ad1dc3e6861601247abf4ac96567/xarray/core/groupby.py#L469

`group` at that stage should have values
```
['a', 'a', 'b', 'b', 'c', 'c']
```

@mschrimpf Can you try that and send in a PR?","{""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,1318992926
https://github.com/pydata/xarray/issues/6836#issuecomment-1212214557,https://api.github.com/repos/pydata/xarray/issues/6836,1212214557,IC_kwDOAMm_X85IQO0d,2448579,2022-08-11T16:28:10Z,2022-08-11T16:36:35Z,MEMBER,"@benbovy I tracked this down to

``` python
>>> mda.one.to_index()
# v2022.06.0
MultiIndex([('a', 0),
            ('a', 1),
            ('b', 0),
            ('b', 1),
            ('c', 0),
            ('c', 1)],
           names=['one', 'two'])

# v2022.03.0
Index(['a', 'a', 'b', 'b', 'c', 'c'], dtype='object', name='x')
```

We call `to_index` here in `safe_cast_to_index`:
https://github.com/pydata/xarray/blob/f8fee902360f2330ab8c002d54480d357365c172/xarray/core/utils.py#L115-L140

Not sure if the fix should be only in the GroupBy specifically or more generally in `safe_cast_to_index`

The GroupBy context is
https://github.com/pydata/xarray/blob/f8fee902360f2330ab8c002d54480d357365c172/xarray/core/groupby.py#L434","{""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,1318992926