Notes disappearing right after creation
I observe same behaviour on one of my laptops (both use gmail, but different accounts). One of the solutions is to switch to iCloud sync. Notes stop disappearing, but you can't edit anything while notes are being updated... which happens each time you update something. Apple surely was in the rush to roll out that one. 

I haven't figured out the exact reason yet though.. Played with Notes folder IMAP settings to no avail.