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
690268366,MDU6SXNzdWU2OTAyNjgzNjY=,4397,Unstack non-index coordinate,38673295,open,0,,,2,2020-09-01T16:04:51Z,2022-04-28T15:13:37Z,,NONE,,,,"**What happened**:

Non-indexed coordinates are not unstacked correctly. The example below should be self-explanatory. 

In the example below, I create a surface of the torus which should be used for some calculation for each point - therefore I have to use multi-index. The surface is given by the set of (R, Z) -- indexed by `pol_idx` -- which is the same for each toroidal angle `phi`. At the end of the day I would like to have the mesh of ('phi', 'pol_idx'). 

**What you expected to happen**:

`ds.stack(multiindex=(""a"", ""b"")`).unstack(""multindex"")` should return `ds`. 

**Minimal Complete Verifiable Example**:

```python
theta = np.linspace(0, np.pi)
R0 = 1

grid = xr.Dataset(coords={""R"": (""pol_idx"", R0 * np.cos(theta)), 
                          ""Z"": (""pol_idx"", R0 * np.sin(theta)), 
                          ""phi"": np.linspace(0, 2*np.pi, N_phi_points, endpoint=False)})
> <xarray.Dataset>
> Dimensions:  (phi: 256, pol_idx: 50)
> Coordinates:
>    R        (pol_idx) float64 1.0 0.9979 0.9918 0.9816 ... -0.9918 -0.9979 -1.0
>    Z        (pol_idx) float64 0.0 0.06407 0.1279 ... 0.1279 0.06407 1.225e-16
>  * phi      (phi) float64 0.0 0.02454 0.04909 0.07363 ... 6.21 6.234 6.259
> Dimensions without coordinates: pol_idx
> Data variables:
>   *empty*

grid = grid.stack(index=(""pol_idx"", ""phi""))

> <xarray.Dataset>
> Dimensions:  (index: 12800)
> Coordinates:
>     R        (index) float64 1.0 1.0 1.0 1.0 1.0 ... -1.0 -1.0 -1.0 -1.0 -1.0
>     Z        (index) float64 0.0 0.0 0.0 0.0 ... 1.225e-16 1.225e-16 1.225e-16
>   * index    (index) MultiIndex
>   - pol_idx  (index) int64 0 0 0 0 0 0 0 0 0 0 ... 49 49 49 49 49 49 49 49 49 49
>   - phi      (index) float64 0.0 0.02454 0.04909 0.07363 ... 6.21 6.234 6.259
> Data variables:
>     *empty*

# Here I would like to perform some calculation on the prepared grid over the multiindex.

grid = grid.unstack(""index"")

> <xarray.Dataset>
> Dimensions:  (phi: 256, pol_idx: 50)
> Coordinates:
>   R        (pol_idx, phi) float64 1.0 1.0 1.0 1.0 1.0 ... -1.0 -1.0 -1.0 -1.0
>   Z        (pol_idx, phi) float64 0.0 0.0 0.0 ... 1.225e-16 1.225e-16
>  * pol_idx  (pol_idx) int64 0 1 2 3 4 5 6 7 8 9 ... 41 42 43 44 45 46 47 48 49
>   * phi      (phi) float64 0.0 0.02454 0.04909 0.07363 ... 6.21 6.234 6.259
> Data variables:
>     *empty*

```

**Comment**:

I am not sure whether this is a bug. However, I find this behaviour counterintuitive. 

**Environment**:

<details><summary>Output of <tt>xr.show_versions()</tt></summary>
INSTALLED VERSIONS
------------------
commit: None
python: 3.7.4 (default, Aug 13 2019, 20:35:49) 
[GCC 7.3.0]
python-bits: 64
OS: Linux
OS-release: 4.15.0-112-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8
libhdf5: 1.10.4
libnetcdf: 4.6.1

xarray: 0.14.1
pandas: 0.25.1
numpy: 1.17.2
scipy: 1.3.1
netCDF4: 1.4.2
pydap: None
h5netcdf: 0.7.4
h5py: 2.9.0
Nio: None
zarr: None
cftime: 1.0.4.2
nc_time_axis: None
PseudoNetCDF: None
rasterio: None
cfgrib: None
iris: None
bottleneck: 1.2.1
dask: 2.5.2
distributed: 2.5.2
matplotlib: 3.1.3
cartopy: None
seaborn: 0.9.0
numbagg: None
setuptools: 41.4.0
pip: 19.2.3
conda: 4.8.3
pytest: 5.2.1
IPython: 7.8.0
sphinx: 2.2.0
</details>
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