I managed to get my missing notebook restored with help from Evernote Tech Support, eventually. Here are the instructions that I followed, my own details replaced with <>. Hope this helps.
I reviewed your account and it appears that the references missing notes may not have properly synced to the servers from the Evernote for Windows application v 6.26 prior to update (between 2020-09-22 and 2020-10-21). With that being said, there should still be the local database from your previous version stored on your PC, and we should still be able to recover the missing content from there.
1. First, lets make a copy of your old database folder, just to be on the safe side (this is important!):
Press the WIN + R keys to open the Run window.
Copy and paste this string into the run window: C:\Users\<name>\Evernote\
RIght-click on the Databases folder and select Copy (CTRL+C) to copy that entire folder to the clipboard
Paste the copy of the folder to your desktop CTRL+V
2. Second, let's download and install the legacy Evernote for Windows client (please DO NOT sign in!).
The Evernote Legacy app can be installed side-by-side with the new Evernote app, and is denoted by a gray Evernote icon. You don't need to delete or uninstall the new app.
Click on the following link https://cdn1.evernote.com/win6/public/Evernote_18.104.22.16898.exe
Open the downloaded file to begin the installation.
3. Now lets extract notebooks from your old database file using the legacy app and locate the older version of your '<missing notebook>' notebook.
Open the legacy Evernote for Windows app you installed in previous step (gray icon) so that you are looking at login screen (please DO NOT sign in!)
Select: Ctrl+Shift+E while viewing the login screen.
Select <account> and click OK
This will create a new folder on your Desktop that contains a separate .enex file for each notebook within that account extracted from that local database. Locate the notebook title '<missing notebook>' (as well as any other notebook that appear to be missing in the new version 10).
Quit the Legacy app (Evernote > Exit)
After that you have two options to import the notes using your new Evernote desktop app (v10):
RIght-click the [notebook_title].enex (e.g. Journal.enex) and select Open with > Evernote (green icon) to open it in the new Evernote app. This will automatically import those notes into a notebook titled (Imported)[notebook_title]
Select: File > Import > Evernote Export Files, and select the .enex file.