https://www.theverge.com/2018/7/10/17552858/netflix-smart-downloads-episodes-shows-android

Today, Netflix released a new feature called “smart downloads” on Android phones and tablets. It will automatically delete downloaded episodes you’ve already watched and replace them with the next episode when you’re connected to Wi-Fi, helping to maximize storage space.

So, for example: if you’ve downloaded episodes 1 through 3 of a show, and you finish watching episode 1, Netflix will delete episode 1 and download episode 4. Downloads won’t start when you’re on mobile data, so it won’t eat up your data.