This just began happening to me last night though I realize it is somewhat of a dated issue.
In case this might help someone, if you find your notes disappearing on your phone, they might be hiding in a folder on your Yahoo mail account (access via Internet not on the phone itself).
I didn't even realize my notes were linked to my Yahoo account until I happened to notice an account tab on my Notes app page. If you do more searchin in the community, you'll find instructions on how to sync/unsync Notes to email accounts (under settings, mail).
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