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
1959816045,I_kwDOAMm_X8500Gtt,8368,"to_netcdf: Unexpected drop of ""units"" attribute of attached ""bounds""",15173535,open,0,,,4,2023-10-24T18:15:05Z,2024-04-09T11:11:20Z,,NONE,,,,"### What happened?

When writing a Dataset to netcdf, any DataArrays that are linked as bounds through another variables attrs['bounds'] entry, have their (specifically) 'units' attribute dropped inside the written netcdf file.

See example

### What did you expect to happen?

Units attribute to be written to the netcdf file.

### Minimal Complete Verifiable Example

```Python
import numpy as np
import xarray as xr

# Create a new Dataset
ds = xr.Dataset()

# Add the x variable, Specify 'x_bnds' as bounds, defined later.
ds['x'] = xr.DataArray(np.arange(10), dims='x', attrs={'units':'m', 'bounds':'x_bnds'})

# Bounds require an extra dimension equal to number of vertices.
ds['nv'] = xr.DataArray(np.r_[0, 1], dims='nv')

# Add the actual bounding values for variable x.
ds['x_bnds'] = xr.DataArray(np.squeeze(np.dstack([np.arange(10)-0.5, np.arange(10)+0.5])),  
                            dims=['x', 'nv'],
                            attrs={'test':4, 'units':'m', })

print('Units is attached to the bounds in the dataset before writing', 'units' in ds['x_bnds'].attrs)

# Write to netcdf file
ds.to_netcdf('tmp.nc', format='netcdf4', engine='netcdf4')

# Open the dataset and check x_bnds attrs.  units is dropped.
new = xr.open_dataset('tmp.nc')
print(new['x_bnds'].attrs)

# Confirm that units were never written to the file.
!h5dump -d /x_bnds tmp.nc
```


### 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](https://mybinder.org/v2/gh/pydata/xarray/main?urlpath=lab/tree/doc/examples/blank_template.ipynb), 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

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.11.5 (main, Sep 11 2023, 08:19:27) [Clang 14.0.6 ]
python-bits: 64
OS: Darwin
OS-release: 21.6.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: ('en_US', 'UTF-8')
libhdf5: 1.12.2
libnetcdf: 4.9.3-development

xarray: 2023.10.1
pandas: 2.1.1
numpy: 1.26.1
scipy: 1.11.3
netCDF4: 1.6.4
pydap: None
h5netcdf: None
h5py: 3.10.0
Nio: None
zarr: None
cftime: 1.6.3
nc_time_axis: None
PseudoNetCDF: None
iris: None
bottleneck: None
dask: None
distributed: None
matplotlib: 3.8.0
cartopy: None
seaborn: None
numbagg: None
fsspec: None
cupy: None
pint: None
sparse: None
flox: None
numpy_groupies: None
setuptools: 68.0.0
pip: 23.3
conda: None
pytest: None
mypy: None
IPython: 8.16.1
sphinx: 7.2.6

</details>
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