distributed: Task stream does not render in get_task_stream or performance_report HTML reports

What happened: HTML reports from get_task_stream and performance_report contain grid and controls for the task stream output, but not task content (colored blocks), while the live dashboard (8787/status) does show task stream contents

What you expected to happen: contents in HTML report similar to live dashboard output

Minimal Complete Verifiable Example:

setup:

conda create -n dask-test python==3.8 -c conda-forge
conda install dask distributed ipython -c conda-forge

code:

from dask.distributed import LocalCluster
c = LocalCluster()

from dask.distributed import Client
client = Client(c)

def square(x):
    return x*x

import distributed

with distributed.get_task_stream(plot='save', filename="task-stream.html") as ts:
    f = client.map(square, [x for x in range(100)])
    distributed.wait(f) # ensure completion per @jrbourbeau's point below

with distributed.performance_report(filename="perf.html") as ts:
    f2 = client.map(square, [x for x in range(200,300)])
    distributed.wait(f2)

Compare html outputs to dashboard task stream

Environment:

• Dask version: 2.30.0, distributed 2.30.0
• Python version: 3.8
• Operating System: MacOS (for repro example) but issue appears on Ubuntu and Coiled Cloud as well
• Install method (conda, pip, source): conda, as above
• Issue tested in Chrome 85.0.4183.121 and Firefox 81.0.1