Here's how you write a scene that ensures the characters in the film are on the same page as the audience...
Predator 1: We, the audience, know that the Predator sees using heat. Dutch doesn't and is being chased by the Predator. Dutch falls into the water, but the Predator keeps coming. Dutch gets covered in mud, the Predator approaches, but does not attack. Dutch is confused. The Predator looks directly at Dutch. Nothing happens. Cut to the Predator's POV and we see that the Predator can no longer distinguish Dutch. The Predator targets another heat signature and attacks, but it's just a rodent. Dutch now understands... when's he's covered in mud, the Predator cannot see him.
It's brilliantly simply and perfectly logical...because Dutch is the thing being seen and then not seen. Prey gets it wrong by not doing something similar with Naru. For her to understand the effect of the herb, she should be seen, but then not seen after consuming some of said herb. That's a logical leap her character could have made.