Hi Linda,
as one of the many posters on this topic who has gone through all the possible solutions I can offer the following ways around this.
Just to review, notes move to your Yahoo mail account and dissappear from the Notes app. This problem apparently also occurs with some people using Gmail as the default Notes email account, although I haven't been able to replicate it but leaves me distrusting using Gmail as the default for Notes. It does not seem to happen if the default is set to iCloud.
Possible solutions -
As you noted, the notes do not appear in the Yahoo account in the Apple Mail app, so you can install the Yahoo mail app, keep using Apple Notes for making notes, and then access any that dissappear from the Notes app via the Yahoo Mail app. This works but seems a bit counter productive to have 2 mail apps on your iPhone, but at least it works without trouble, is all free and not subject to any additonal charges or limits on data.
Set iCloud as the default email account in Notes. I personally do not wish to entrust all my information to iCloud or one provider.
Install a 3rd party notes app. There are quite a few if you look on the app store.
I've tried a few and have settled on Evernote, which is free up to a certain useage (60mb per month upload iirc) but has much more flexibility than Apple Notes. Yes, it means installing a second note app, but this and many of the 3rd party note apps offer many more features than the Apple Notes app.
HTH