Jupyterlab 3: Renderer broken inside fresh env with extensions properly installed

Thanks for having detailed troubleshooting docs for Jupyterlab issues, however I'm unable to use the plotly_mimetype renderer (blank plot) in a fresh Jupyterlab (3.0.5) conda environment having followed the troubleshooting and getting started guides almost to the letter. The Plotly express plot works if I downgrade to an early 2.x jupyterlab version. This point in particular makes me suspicous that I have encountered an issue, perhaps related to Lab 3.x

Have I somehow missed something? Using Mac OS 10.14.6

Server

1. conda create -n jlab-server jupyterlab nodejs ipywidgets>=7.5
2. jupyter labextension install jupyterlab-plotly@4.14.3
3. jupyter labextension install @jupyter-widgets/jupyterlab-manager plotlywidget@4.14.3
4. jupyter lab build

Kernel
conda install -c plotly plotly=4.14.3

Both jupyter labextension list and the lab UI confirm that these extensions are installed. I'm not confusing server and kernel environments.

$ jupyter labextension list
JupyterLab v3.0.5
/Users/bede/conda/envs/jlab-server/share/jupyter/labextensions
        @jupyter-widgets/jupyterlab-manager v3.0.0 enabled OK (python, jupyterlab_widgets)

Other labextensions (built into JupyterLab)
   app dir: /Users/bede/conda/envs/jlab-server/share/jupyter/lab
        jupyterlab-plotly v4.14.3 enabled OK
        plotlywidget v4.14.3 enabled OK

Reproduce el gráfico en blanco con la configuración del servidor JupyterLab 3.0.5, las extensiones y el entorno del kernel Plotly 4.14.3 indicados. Comienza con las guías enlazadas de resolución de problemas y primeros pasos; después, compara la configuración funcional de JupyterLab 2.x con la lista de extensiones mostrada en el informe. Se considera terminado cuando se haya identificado el problema de compatibilidad con JupyterLab 3 y confirmado que plotly_mimetype se renderiza correctamente en un entorno nuevo.