This is consistent with Oreki's past performance. During the titular case he also didn't predict everything, although back then he suspected it himself. I think this was also psychologically sound, including forgetting the rope. He isn't trained to solve criminal cases (or rather, mysteries), which was also evident in the fact he processed it all only inside his head, instead of using the laptop or a physical board or whatever to make sure he's not missing anything. Maybe the critical factor here is that he was praised too much and got careless. He's talented enough to process information and think out of the box, but he will still fall victim to human errors.

But then again, let's not forget this is a movie and not a real case. Ultimately it's the original author's fault to not finish it. If another person does, it will become that other person's story instead, and there's nothing wrong about it anymore at that point, details included or forgotten. You just have to accept that. The bad acting certainly didn't help and only supported Oreki's version. To be cynical, Ibara made the mistake of not allowing Oreki is get past this whole thing.