I encountered this same issue and fortunately was able to get (most) of my notes back because my most recent iCloud backup for the iPad was prior to when I turned on iCloud Sync for notes.
Issue:
I created new notes on my iPad while the "Sync this iPad" toggle for Notes within Apps using iCloud was off. When I switched on the toggle, all the notes created on my iPad were deleted and replaced by the notes from my iPhone/in iCloud.
APPLE NEEDS TO FIND A SOLUTION FOR THIS - or at the very least provide a warning that the newly created notes could be deleted.
Solution:
- In Settings on your iPad, click your name (enter iCloud settings)
- Click iCloud
- Scroll to DEVICE BACKUPS, click iCloud Backup
- Under ALL DEVICE BACKUPS, click the latest iPad backup and confirm the Last Backup was saved before you switched on the Sync this iPad toggle. For me, my last backup was yesterday.
Next,
- Erase all content and settings
- Restore iPad from latest backup (you will lose anything done after that most-recent backup - for me this was worth it to get the notes back)
- Notes created on iPad should be there. Immediately copy them onto your phone or Google Drive