Actually, the last version of Now TV that works on Firestick is apparently 10.4.1 though it brings it's own set if issues and on further reading, it may not work.
I don't use nowtv myself so haven't tried this but there might be a fix for this, might not.
I have just bought an Amazon Firestick and I can watch many NOW TV programmes, even recent ones like the Undoing, using the NOW TV app that comes with the Firestick and my NOW TV Entertainment Pass. However, I cannot watch other programmes (e.g. Billions), which I usually cast to my TV using my mobile phone, a Chromecast device and NOW TV pass. I can only access this other content when using the Firestick by purchasing each episode or season. Is this the Now TV and Amazon policies clashing or a set-up problem?
Sounds like you are you using the Fire Sticks built in search, which at the moment does not include Now TV content, so you are getting results from Prime Video.
You will have to use the search function within the Now TV app. Once the app is open, long press the Select button on your Fire Stick remote, which will bring up a Sub-Menu with the Search & TV Guide options.
Hope that helps 🙂