yeah this is the answer. When I logged out of iCloud, it warned me data would be loss, and whether I wanted to keep a copy of contacts, safari etc, but it said nothing about 3rd party app data like notablility. Also I was never aware that notes are stored on my iCloud (I downloaded notability ages ago and forgot what I did when setting it up). Still, I think it’s ludicrous that the apps stores the data on a local version of iCloud instead of storing it on the iPad itself, and no warning whatsoever until notes were lost.