This is only seen on the Chromecast which supports Google TV
The Google TV App on Android and the web work perfectly fine
The issue seems likely with Netflix rather than Google
Google TV is one centralized app from Google that curates and displays content from multiple streaming services such as Netflix, Amazon Prime, Disney+Hotstar, and many more. You can add content to your watchlist from multiple sources at one central location. The Google TV app also suggests and lets you search for content from across apps and services.
As spotted by 9to5google, the notable thing is that you cannot add or remove Netflix Originals to your Google TV watchlist on the Chromecast with Google TV(Sabrina). The feature still works with the Google Watchlist found in Google search, if you try to add or remove the content on the web or the Google TV app on Android the feature works completely fine.
Netflix has a huge collection of Original TV shows and movies that are quite popular, and users who have invested in it would like to add these titles to their own personal list on the Google TV app which unfortunately is not possible at the moment.
Although you cannot add or remove the Netflix Originals on the Chromecast, you would still be able to access the content without any hindrance. For some reason, this content is directly blocked on Chromecast and only for Netflix Originals.
The Google spokesperson commented the following to 9to5Google “With Google TV, our goal is to bring the best of our search and discovery features across your subscriptions to your favourite devices. We work with each content partner to enable these entertainment experiences, and the level of integration will vary by partner.”
The issue seems to be with the support of Netflix rather than Google, hopefully, this would eventually be resolved soon.