When a notebook file name is clicked on in the cloud directory on the right one can watch cells get filled in as separate chunks.
Eventually the entire notebook is ready. But not all the time. The cloud lost some cells. The in and out placeholders are there but there's no code or any old output. .
Stranger yet, when the notebook is downloaded to my PC all the cells are present but restarting the session doesn't fix the cloud version.
It looks like edits of the notebook were recorded in the source but not in the displayed version.