Ok, I use notes to record things I might forget. So it took me a while to realize they were disappearing. My wife said hers were disappearing, but I chalked that up to user error until I realized mine were going too. The timing seems random. Some notes stayed a while, others seemed to go quickly. Only today when I realized I only had one note from the last 8 months was I certain something was wrong.
I found this thread and as I read it took some actions that seems to have corrected my problem to some extent.
I went to yahoo in a browser and saw all my notes there in a folder. Which made me feel better.
I went to settings-mail,contacts,calendars selected my yahoo account at the top under accounts.
And turned off notes. Hoping to stop more from being removed.
Reading more, I quickly turned it back on so I would have them all disappear.
Strangely, many (but not all) of my missing notes returned on my phone. I then switched the default under settings-notes to "on my phone" so that all new notes would not be yahoo notes. I think turning it off and back on forces yahoo to resync.
I think this will stop the bleeding. The number of yahoo notes should not change. I need to look into using iCloud to back them up. And I need to identify the still missing notes and transfer them back.
Hope thus helps.