Widget state not persisted after kernel restart

[acbokade]

I have created a custom widget following this tutorial.
I am using ipywidgets == 8.0.6 and jupyterlab == 3.5.0.
By referring to this suggestion, I used the option ' Save Widget State Automatically'. But after the kernel restart, I am getting the 'Error displaying widget: model not found' error.
I am creating a different custom widget but for simplicity, I have used the tutorial example custom widget.
The use case I want to achieve is the same though; I want to persist the widget state in the Jupyter lab.
Is there anything I am missing?

[gutow]

I just noticed this behavior when testing my own widgets built using ipywidgets in the version 7 notebook with ipwidgets ~=8. Everything works, but the widget state is not saved. This definitely worked in the past.

[gutow]

The widget state is still there and saved by JLab. Opening the notebook in NBClassic running against the Jupyter Server 2.7.0 shows the widget states and correctly indicates broken connections for widgets that talk to the kernel. Somehow JLab/Notebook 7 does not find the widget states in Notebooks even though it saves them.