What happened?

From https://github.com/pydata/xarray/discussions/8951

if d is a MultiIndex-ed dataset with levels (x, y, z), and m is a dataset with a single coord x m.isel(x=d.x) builds a dataset with a MultiIndex with levels (y, z). This seems like it should work.

cc @benbovy

What did you expect to happen?

No response

Minimal Complete Verifiable Example

```Python import pandas as pd, xarray as xr, numpy as np

xr.set_options(use_flox=True)

test = pd.DataFrame() test["x"] = np.arange(100) % 10 test["y"] = np.arange(100) test["z"] = np.arange(100) test["v"] = np.arange(100)

d = xr.Dataset.from_dataframe(test) d = d.set_index(index = ["x", "y", "z"]) print(d)

m = d.groupby("x").mean() print(m)

print(d.xindexes) print(m.isel(x=d.x).xindexes)

xr.align(d, m.isel(x=d.x))

res = d.groupby("x") - m

print(res)

```

<xarray.Dataset> Dimensions: (index: 100) Coordinates: * index (index) object MultiIndex * x (index) int64 0 1 2 3 4 5 6 7 8 9 0 1 2 ... 8 9 0 1 2 3 4 5 6 7 8 9 * y (index) int64 0 1 2 3 4 5 6 7 8 9 ... 90 91 92 93 94 95 96 97 98 99 * z (index) int64 0 1 2 3 4 5 6 7 8 9 ... 90 91 92 93 94 95 96 97 98 99 Data variables: v (index) int64 0 1 2 3 4 5 6 7 8 9 ... 90 91 92 93 94 95 96 97 98 99 <xarray.Dataset> Dimensions: (x: 10) Coordinates: * x (x) int64 0 1 2 3 4 5 6 7 8 9 Data variables: v (x) float64 45.0 46.0 47.0 48.0 49.0 50.0 51.0 52.0 53.0 54.0 Indexes: ┌ index PandasMultiIndex │ x │ y └ z Indexes: ┌ index PandasMultiIndex │ y └ z ValueError...

MVCE confirmation

• [x] Minimal example — the example is as focused as reasonably possible to demonstrate the underlying issue in xarray.
• [x] Complete example — the example is self-contained, including all data and the text of any traceback.
• [x] Verifiable example — the example copy & pastes into an IPython prompt or Binder notebook, returning the result.
• [x] New issue — a search of GitHub Issues suggests this is not a duplicate.
• [x] Recent environment — the issue occurs with the latest version of xarray and its dependencies.

Relevant log output

No response

Anything else we need to know?

No response

Environment