problem appending to zarr on GCS when using json token

What happened:
Appending a toy dataset to an existing zarr store in GCS along the time dimension leaves the store unchanged.

What you expected to happen:
The store to double in length, because I was appending a dataset with a length of 3 in the time dimension, to another dataset of the same size.

Minimal Complete Verifiable Example:
To reproduce fully you will need the token, but maybe people can try using their own token.

import fsspec
import xarray as xr
import json
import gcsfs


## define a mapper to the ldeo-glaciology bucket
### needs a token
with open('../secrets/ldeo-glaciology-bc97b12df06b.json') as token_file:
    token = json.load(token_file)

mapper = fsspec.get_mapper('gs://ldeo-glaciology/append_test/test5', mode='w', token=token)

## define two simple datasets
ds0 = xr.Dataset({'temperature': (['time'],  [50, 51, 52])}, coords={'time': [1, 2, 3]})
ds1 = xr.Dataset({'temperature': (['time'],  [53, 54, 55])}, coords={'time':  [4, 5, 6]})

## write the fist to bucket
ds0.to_zarr(mapper)
## append the second to the same zarr store
ds1.to_zarr(mapper, mode='a', append_dim='time')

## load the zarr store
ds_both = xr.open_zarr(mapper)

## this is 3, indicating that the append did not work 
len(ds_both.time)

Anything else we need to know?:
It works as expected if you instead write and append to the pangeo scratch bucket, i.e if you replace

with open('../secrets/ldeo-glaciology-bc97b12df06b.json') as token_file:
    token = json.load(token_file)

mapper = fsspec.get_mapper('gs://ldeo-glaciology/append_test/test5', mode='w', token=token)

with

mapper = fsspec.get_mapper('gs://pangeo-scratch/jkingslake/append_test/test3', mode='w', token=None)

It also works as expected if I write and append to a local zarr.

Thanks for your help!

Environment:
https://us-central1-b.gcp.pangeo.io/

Start by running the minimal example with your own GCS token, comparing the authenticated bucket with the pangeo-scratch and local cases described in the issue. Trace the append operation and verify the fix by confirming that the resulting dataset has six time entries, ideally with a regression test for the authenticated GCS case.