Same Problem Here with New Yahoo Mail
I applied the update to Yahoo Mail and as soon as I did, I too, stopped getting the Notifications. Even when I know I have mail and open the app, nothing shows until I pull to refresh. Every so often, one will come through and I think it's been fixed but then they stop again. I had one of the developers email me asking for my Yahoo ID so they can see why they're not pushing but after I sent it to him, I've heard nothing back. It's very frustrating. I started using the stock email app for my Yahoo Mail just to get the notifications so I know when to go into the Yahoo Mail app and pull to refresh.