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
2213636579,I_kwDOAMm_X86D8Wnj,8887,resetting multiindex may be buggy,2448579,open,0,,,1,2024-03-28T16:23:38Z,2024-03-29T07:59:22Z,,MEMBER,,,,"### What happened?

Resetting a MultiIndex dim coordinate preserves the MultiIndex levels as IndexVariables.  We should either reset the indexes for the multiindex level variables, or warn asking the users to do so

This seems to be the root cause exposed by https://github.com/pydata/xarray/pull/8809

cc @benbovy 

### What did you expect to happen?

_No response_

### Minimal Complete Verifiable Example

```Python
import numpy as np
import xarray as xr

# ND DataArray that gets stacked along a multiindex
da = xr.DataArray(np.ones((3, 3)), coords={""dim1"": [1, 2, 3], ""dim2"": [4, 5, 6]})
da = da.stack(feature=[""dim1"", ""dim2""])

# Extract just the stacked coordinates for saving in a dataset
ds = xr.Dataset(data_vars={""feature"": da.feature})
xr.testing.assertions._assert_internal_invariants(ds.reset_index([""feature"", ""dim1"", ""dim2""]), check_default_indexes=False) # succeeds
xr.testing.assertions._assert_internal_invariants(ds.reset_index([""feature""]), check_default_indexes=False) # fails, but no warning either
```

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