You might find some of your movies or show on Netflix downloads are gone surprisingly. The downloaded videos also have an expiration time, but that differs from video to video relying on the licensing deal Netflix has with the creator of that video.
Users are only alerted about an expiring download 7 days prior to the expiration date and in case any downloaded video is deleted from the platform, the download expires instantly. In addition to this, some titles have a cap on how many times they can be downloaded every year.