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I noticed something similar recently. I purchased "The Fast and the Furious" on Prime quite some time ago. Recently I searched for it, and the only entry that shows up shows that I would have to pay to watch it. WTF? So I go into my Video Library and browse to that title, and sure enough it says "You purchased this" and I can watch it.

I assume that in my case, the issue is that certain titles are listed more than once (for whatever bizarre reason) and the particular instance I bought is no longer available. OR, a less charitable take would be that Amazon do this intentionally to try to trick people into paying a second time for something they've already purchased...

Not sure what I believe yet, and not sure if this relates to your issue or not. So take this FWIW.