It is a bug, but the bug belongs to Amazon Prime, not Roku. They provide the information to Roku about their content for the search functions, and in this case they seem to have their information wrong about free content. If you instead go to the Prime Video app and search for that movie, do you find a free version available?
OK, I think I see what the issue is. Look at that image, and directly below the title it says "Coming to Prime in 4 days". I went to Amazon's web site and see the same thing. So if you are a Prime member it appears it will be free in four days from today (so July 25?). That tells me Amazon didn't code the title properly in the database that Roku uses to retrieve the information.