Netcdf char array not being decoded to string in compound dtype

Code Sample, a copy-pastable example if possible

script_nc_file = "bolo_geom_fromscript.nc"
with xr.open_dataset(script_nc_file, group='/bolo/sxd', concat_characters=True) as ds:
    ds = ds.copy(deep=True)
print(ds.slits)
<xarray.DataArray 'slits' (slit_no: 4)>
array([ ([b'B', b'o', b'l', b'o', b'm', b'e', b't', b'e', b'r', b'S', b'l', b'i', b't', b'', b'', b'', b'', b'', b'', b'', b'', b'', b'', b'', b'', b'', b'', b'', b'', b''], [b'M', b'A', b'S', b'T', b'-', b'U', b' ', b'S', b'X', b'D', b' ', b'-', b' ', b'O', b'u', b't', b'e', b'r', b' ', b'S', b'l', b'i', b't', b' ', b'1', b'', b'', b'', b'', b''], 1, (-0.06458486,  0.21803484, -0.97380162), ( 0.95881973,  0.28401534,  0.), (-0.52069675,  1.77104629, -1.564     ),  0.005,  0.005, [b'M', b'A', b'S', b'T', b'-', b'U', b' ', b'S', b'X', b'D', b' ', b'-', b' ', b'O', b'u', b't', b'e', b'r', b' ', b'S', b'l', b'i', b't', b' ', b'1', b'', b'', b'', b'', b''], 0),
       ([b'B', b'o', b'l', b'o', b'm', b'e', b't', b'e', b'r', b'S', b'l', b'i', b't', b'', b'', b'', b'', b'', b'', b'', b'', b'', b'', b'', b'', b'', b'', b'', b'', b''], [b'M', b'A', b'S', b'T', b'-', b'U', b' ', b'S', b'X', b'D', b' ', b'-', b' ', b'O', b'u', b't', b'e', b'r', b' ', b'S', b'l', b'i', b't', b' ', b'2', b'', b'', b'', b'', b''], 1, (-0.16038567,  0.54145294, -0.82529095), ( 0.95881973,  0.28401534,  0.), (-0.5278879 ,  1.76891617, -1.564     ),  0.005,  0.005, [b'M', b'A', b'S', b'T', b'-', b'U', b' ', b'S', b'X', b'D', b' ', b'-', b' ', b'O', b'u', b't', b'e', b'r', b' ', b'S', b'l', b'i', b't', b' ', b'2', b'', b'', b'', b'', b''], 1),
       ([b'B', b'o', b'l', b'o', b'm', b'e', b't', b'e', b'r', b'S', b'l', b'i', b't', b'', b'', b'', b'', b'', b'', b'', b'', b'', b'', b'', b'', b'', b'', b'', b'', b''], [b'M', b'A', b'S', b'T', b'-', b'U', b' ', b'S', b'X', b'D', b' ', b'-', b' ', b'U', b'p', b'p', b'e', b'r', b' ', b'S', b'l', b'i', b't', b' ', b'3', b'', b'', b'', b'', b''], 1, (-0.26470454,  0.89362754, -0.36243804), ( 0.95881973,  0.28401534,  0.), (-0.31231469,  1.06756025, -1.57072314),  0.005,  0.005, [b'M', b'A', b'S', b'T', b'-', b'U', b' ', b'S', b'X', b'D', b' ', b'-', b' ', b'U', b'p', b'p', b'e', b'r', b' ', b'S', b'l', b'i', b't', b' ', b'3', b'', b'', b'', b'', b''], 2),
       ([b'B', b'o', b'l', b'o', b'm', b'e', b't', b'e', b'r', b'S', b'l', b'i', b't', b'', b'', b'', b'', b'', b'', b'', b'', b'', b'', b'', b'', b'', b'', b'', b'', b''], [b'M', b'A', b'S', b'T', b'-', b'U', b' ', b'S', b'X', b'D', b' ', b'-', b' ', b'U', b'p', b'p', b'e', b'r', b' ', b'S', b'l', b'i', b't', b' ', b'4', b'', b'', b'', b'', b''], 1, (-0.19640032,  0.66303636,  0.72236396), ( 0.95881973,  0.28401534,  0.), (-0.31950584,  1.06543013, -1.57072314),  0.005,  0.005, [b'M', b'A', b'S', b'T', b'-', b'U', b' ', b'S', b'X', b'D', b' ', b'-', b' ', b'U', b'p', b'p', b'e', b'r', b' ', b'S', b'l', b'i', b't', b' ', b'4', b'', b'', b'', b'', b''], 3)], 
      dtype={'names':['Object_type','ID','Version','basis_1','basis_2','centre_point','width','height','slit_id','slit_no'], 'formats':[('S1', (30,)),('S1', (30,)),'<i4',[('x', '<f8'), ('y', '<f8'), ('z', '<f8')],[('x', '<f8'), ('y', '<f8'), ('z', '<f8')],[('x', '<f8'), ('y', '<f8'), ('z', '<f8')],'<f8','<f8',('S1', (30,)),'<i4'], 'offsets':[0,30,60,64,88,112,136,144,152,184], 'itemsize':192, 'aligned':True})
Coordinates:
  * slit_no  (slit_no) int64 0 1 2 3
Attributes:
    units: m

Problem description

When opening the attached dataset ,the char arrays in the compound dtype are not being converted into strings, despite concat_characters=True.
bolo_geom_fromscript.nc.zip

Expected Output

The char arrays should be converted into strings (or at the very least, bytes if an encoding is not present):

import netCDF4 as nc
longest_str = np.max([ds.slits.values[field].shape[-1] for field in ds.slits.values.dtype.fields 
                      if ds.slits.values[field].dtype.kind in ('S', 'U')])
str_dtype = '<U{}'.format(longest_str)
cartesian_coord = np.dtype([('x', np.float64), ('y', np.float64), ('z', np.float64)])
aperture_dtype_str = np.dtype([('Object_type', str_dtype), 
                               ('ID', str_dtype), ('Version', np.int32),
                               ('basis_1', cartesian_coord), ('basis_2', cartesian_coord), 
                               ('centre_point', cartesian_coord),
                               ('width', np.float64), ('height', np.float64),
                               ('slit_id', str_dtype), ('slit_no', np.int32)])
ds['slits_str'] = xr.DataArray(np.empty(ds.slits.size, aperture_dtype_str), 
                               coords=[('slit_no', ds.coords['slit_no'])])
for key in ds.slits.values.dtype.fields:
    if key in ('Object_type', 'ID', 'slit_id'):
        string_key = nc.chartostring(ds.slits.values[key])
        ds.slits_str.values[key] = string_key
    else:
        ds.slits_str.values[key] = ds.slits.values[key]
print(ds.slits_str)

<xarray.DataArray 'slits_str' (slit_no: 4)>
array([ ('BolometerSlit', 'MAST-U SXD - Outer Slit 1', 1, (-0.06458486,  0.21803484, -0.97380162), ( 0.95881973,  0.28401534,  0.), (-0.52069675,  1.77104629, -1.564     ),  0.005,  0.005, 'MAST-U SXD - Outer Slit 1', 0),
       ('BolometerSlit', 'MAST-U SXD - Outer Slit 2', 1, (-0.16038567,  0.54145294, -0.82529095), ( 0.95881973,  0.28401534,  0.), (-0.5278879 ,  1.76891617, -1.564     ),  0.005,  0.005, 'MAST-U SXD - Outer Slit 2', 1),
       ('BolometerSlit', 'MAST-U SXD - Upper Slit 3', 1, (-0.26470454,  0.89362754, -0.36243804), ( 0.95881973,  0.28401534,  0.), (-0.31231469,  1.06756025, -1.57072314),  0.005,  0.005, 'MAST-U SXD - Upper Slit 3', 2),
       ('BolometerSlit', 'MAST-U SXD - Upper Slit 4', 1, (-0.19640032,  0.66303636,  0.72236396), ( 0.95881973,  0.28401534,  0.), (-0.31950584,  1.06543013, -1.57072314),  0.005,  0.005, 'MAST-U SXD - Upper Slit 4', 3)], 
      dtype=[('Object_type', '<U30'), ('ID', '<U30'), ('Version', '<i4'), ('basis_1', [('x', '<f8'), ('y', '<f8'), ('z', '<f8')]), ('basis_2', [('x', '<f8'), ('y', '<f8'), ('z', '<f8')]), ('centre_point', [('x', '<f8'), ('y', '<f8'), ('z', '<f8')]), ('width', '<f8'), ('height', '<f8'), ('slit_id', '<U30'), ('slit_no', '<i4')])
Coordinates:
  * slit_no  (slit_no) int64 0 1 2 3

Output of xr.show_versions()

Start by reproducing the issue with the attached dataset through xarray.open_dataset using concat_characters=True, then trace the character-array handling for compound dtypes. Done means the compound fields are converted to strings or bytes while preserving the other fields and structure shown in the expected output.