SoPM describes Nue's ability as "concealing its true nature by removing its form, sound, and smell, leaving only its behavior". Your mind fills in the blanks, so if it's behavior is particularly obvious you might correctly perceive what it is (the article says you'd see a "mirage-like bird" if it behaves like a bird). Basically, they could see the pieces of wood because they expected to see them floating around the area, so when they see a featureless flying thing they see wood instead. They aren't seeing the actual wood though, they're seeing the illusion of what they expect to see, which happens to be correct. "Mirage-like" wood, that's only how they imagine the wood would be. If the illusion was broken, they would see that all the details are different.
In the case of "showing Rinnosuke an object", he literally cannot see it, and even if he guesses what it is from its behavior, he's still never perceiving it with his eyes. The object is essentially invisible, and I don't think there's any reason to believe his power works on invisible things.
