@pikahika It appears to be plot, whenever he's using his ability to analyze things. You can first see when his eye changes in chapter 5 page 3 where it's normal, then page 4 when it changes as he's then able to identify the B rated corn. It's then back to normal for a while until it changes again on page 8 when he's analyzing more corn.
It's a fairly common trope that when someone's using a power to identify things or analyze things, their eyes change.
In this chapter, he's analyzing the seed of beauty.