Yet another work-around…
Answering what may be a related question here, I did see that @dlukes had mentioned reloading helped get to the correct workspace and so I wondered if it would hep when MyBinder launches don’t take you to the correct notebook. Sure enough when the second Chrome browser window I had opened with the example link I used above failed to open the correct notebook, I was able to issue a hard refresh of the page (command-R on a Mac) and it then opened the correct notebook after the page refreshed.
CAVEAT TO ALL THESE WORK-AROUNDS/SOLUTIONS:
Currently, none of these work-arounds/solutions allow the workspace loading that the ‘Try Jupyter’ JupyterLab demo uses to open properly. (The 'Try ‘Jupyter’ demo is here at Project Jupyter | Try Jupyter. If you go there and click Try JupyterLab to launch https://mybinder.org/v2/gh/jupyterlab/jupyterlab-demo/master?urlpath=lab/tree/demo you’ll see it currently doesn’t load the notebook or panes it should. The repo backing that demonstration is here). That workspace isn’t just a specific notebook but an arrangement of panes in the workspace, including a notebook ‘Lorenz.ipynb’. And currently it is using JupyterLab 3.0.7. I have tried it in both Chrome and Firefox and don’t see this loading via MyBinder properly. And so this goes back to saying there may be an issue with workspaces and MyBinder working as I raised here. Maybe it is just more complex workspaces? If you are lucky, yours isn’t this present issue.