I am having the same issue - my default notebook has over 10,000 notes - starting from 2008 . It is not syncing to the Android app.
I can see it, as expected, in the web browser (Chrome running on Linux, Firefox running on Linux) and Windows.
In the Android app, when I go to Settings -> Notebooks -> Default notebook - nothing shows.
Offline Notebooks shows my default notebook in the list - but the toggle to download the default notebook does not appear as green and does not stay selected if I click on it.