Version: 1.53.0-insider
OS: Darwin x64 20.2.0

Steps to Reproduce:

1. Create an extension using the VSC Notebook API
2. Add event handler to onDidSaveNotebookDocument
3. Save a notebook and this event isn't fired.

This seems to only work when saving an untitled notebook or using the Save As option.
Doesn't fire for regular saves (with & without auto save).

Does this issue occur when all extensions are disabled?: Yes