Chunking causes unrelated non-dimension coordinate to become a dask array

What happened:

Rechunking along an independent dimension causes unrelated non-dimension coordinates to become dask arrays. The dimension coordinates do not seem affected.

I can stick in a synchronous compute on the coordinate to recover, but wanted to be sure this was the expected behavior.

What you expected to happen:

Chunking along an unrelated dimension should not affect unrelated non-dimension coordinates.

Minimal Complete Verifiable Example:

import xarray as xr
import dask.array as da

def print_coords(a, title):
    print()
    print(title)
    for dim in ['x', 'y', 'b']:
        if dim in a.dims or dim in a.coords:
            print('dim:', dim, 'type:', type(a.coords[dim].data))

arr = xr.DataArray(da.zeros((20, 20), chunks=10), dims=('x', 'y'), 
                   coords={'b': ('y', range(100,120)), 
                           'x': range(20), 
                           'y': range(20)})

print_coords(arr, 'Original')

# The following line rechunks independently of b or y.
# Removing this line allows the code to succeed.
arr = arr.chunk({'x': 5})

print_coords(arr, 'After chunking')

arr = arr.sel(y=2)

print_coords(arr, 'After selection')

print()
print('Scalar values:')
print('y=', arr.coords['y'].item())
print('b=', arr.coords['b'].item())  # Sad Panda

Original
dim: x type: <class 'numpy.ndarray'>
dim: y type: <class 'numpy.ndarray'>
dim: b type: <class 'numpy.ndarray'>

After chunking
dim: x type: <class 'numpy.ndarray'>
dim: y type: <class 'numpy.ndarray'>
dim: b type: <class 'dask.array.core.Array'>

After selection
dim: x type: <class 'numpy.ndarray'>
dim: y type: <class 'numpy.ndarray'>
dim: b type: <class 'dask.array.core.Array'>

Scalar values:
y= 2

<stack trace elided>
NotImplementedError: 'item' is not yet a valid method on dask arrays

Environment:

Run the minimal complete verifiable example with DataArray.chunk({'x': 5}) and the subsequent sel(y=2) call to reproduce the dask-backed b coordinate and failing item() access. Trace the chunking and selection entry points, then verify that rechunking an unrelated dimension leaves unrelated non-dimension coordinates usable as before.