Be sure you are looking in the yahoo FOLDER called 'Notes' not the yahoo 'notepad' app. I made this mistake too.
Turning off sync to yahoo resolves the problem of the notes disappearing from the iphone but syncing to yahoo allows me to sync them to both my iphone and my ipad. If I disable the sync I disable this function.
I need to know why it deletes them intermittently, Why not all of the notes, Why there are a couple hundred notes in yahoo. Most of them are notes I deleted from my iphone/ipad notes app.
And how to resolve the issue so the app syncs accross both devices with or without Yahoo.
Thanks,