In talking to Amazon Customer Service, it appears you have to turn it off (or, on and then off again) in the web page under Manage Your Prime Video. I find though, that it resets itself every other time or so and the problem is back. This is so highly annoying. Also, for me it always plays the final episode of Homecoming, even though I've seen it, and it's not on my Watchlist.
Question has not been answered. Autoplay even starts before credits have completed running. The Amazon Prime app on Apple TV contains a Settings option to turn off autoplay, but the Comcast app does not. This is very, very annoying.
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