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
903055859,MDU6SXNzdWU5MDMwNTU4NTk=,5384,"Regression: ""ValueError: cannot unstack dimensions that do not have a MultiIndex"" when unstacking a MultiIndex",404832,closed,0,,,5,2021-05-27T00:34:27Z,2021-05-28T08:28:11Z,2021-05-28T08:28:11Z,CONTRIBUTOR,,,,"<!-- Please include a self-contained copy-pastable example that generates the issue if possible.

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I'm not sure if this is a bug or I'm not using `xarray` correctly, but I used to be able to do this without crashing. The new behavior seems to have been introduced some time between 0.16.2 and 0.18.2.

**What happened**:

```python traceback
Traceback (most recent call last):
  File ""scripts/repro.py"", line 12, in <module>
    ds = ds.unstack(['c'])
  File ""/home/darsh/src/notebooks/build/venv/lib/python3.8/site-packages/xarray/core/dataset.py"", line 4024, in unstack
    raise ValueError(
ValueError: cannot unstack dimensions that do not have a MultiIndex: ['c']
```

**What you expected to happen**:

The code runs without the `ValueError` exception.

**Minimal Complete Verifiable Example**:

```python
from xarray import DataArray, Dataset


a = DataArray([0], dims=['a'])
b = a.stack(b=('a',)).reset_index('b')
c = b.stack({'c': ['b']})

ds = Dataset({'d': DataArray(c.data, dims=['c'])}, coords=c.coords)
print('\nBefore:')
print(ds)

ds = ds.unstack(['c'])
print('\nAfter:')
print(ds)
```

**Anything else we need to know?**:

Here's the full output from the example on 0.18.2:
```

Before:
<xarray.Dataset>
Dimensions:  (c: 1)
Coordinates:
  * c        (c) MultiIndex
  - b        (c) int64 0
    a        (c) int64 0
Data variables:
    d        (c) int64 0
Traceback (most recent call last):
  File ""scripts/repro.py"", line 12, in <module>
    ds = ds.unstack(['c'])
  File ""/home/darsh/src/notebooks/build/venv/lib/python3.8/site-packages/xarray/core/dataset.py"", line 4024, in unstack
    raise ValueError(
ValueError: cannot unstack dimensions that do not have a MultiIndex: ['c']
```
What confuses me is that the `c` dimension is shown as a `MultiIndex`, but it still complains that it doesn't have a `MultiIndex`. Directly unstacking `ds.d` rather than the dataset itself also fails with the same exception.

Oddly, it seems to work if I assign the coordinates after constructing the dataset:
```diff
diff --git a/scripts/repro.py b/scripts/repro.py
index ed2ae7c..d5bd6a3 100644
--- a/scripts/repro.py
+++ b/scripts/repro.py
@@ -5,7 +5,7 @@ a = DataArray([0], dims=['a'])
 b = a.stack(b=('a',)).reset_index('b')
 c = b.stack({'c': ['b']})
 
-ds = Dataset({'d': DataArray(c.data, dims=['c'])}, coords=c.coords)
+ds = Dataset({'d': DataArray(c.data, dims=['c'])}).assign_coords(c.coords)
 print('\nBefore:')
 print(ds)
 

```

With that workaround, or by downgrading to 0.16.2, the example doesn't crash:

```

Before:
<xarray.Dataset>
Dimensions:  (c: 1)
Coordinates:
  * c        (c) MultiIndex
  - b        (c) int64 0
    a        (c) int64 0
Data variables:
    d        (c) int64 0

After:
<xarray.Dataset>
Dimensions:  (b: 1)
Coordinates:
    a        (b) int64 0
  * b        (b) int64 0
Data variables:
    d        (b) int64 0
```


**Environment**:

<details><summary>Output of <tt>xr.show_versions()</tt></summary>

<!-- Paste the output here xr.show_versions() here -->

INSTALLED VERSIONS
------------------
commit: None
python: 3.8.0 (default, Feb 25 2021, 22:10:10) 
[GCC 8.4.0]
python-bits: 64
OS: Linux
OS-release: 5.4.0-73-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: ('en_US', 'UTF-8')
libhdf5: None
libnetcdf: None

xarray: 0.18.2
pandas: 1.2.4
numpy: 1.20.3
scipy: 1.6.3
netCDF4: None
pydap: None
h5netcdf: None
h5py: None
Nio: None
zarr: None
cftime: None
nc_time_axis: None
PseudoNetCDF: None
rasterio: None
cfgrib: None
iris: None
bottleneck: None
dask: 2021.05.0
distributed: None
matplotlib: 3.4.2
cartopy: None
seaborn: None
numbagg: None
pint: 0.17
setuptools: 39.0.1
pip: 21.1.1
conda: None
pytest: 6.2.4
IPython: 7.23.1
sphinx: None
None


</details>
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