P0420 Code only when there is less than half tank of fuel

Hello all. This is my first post.
I searched the forum to look for similar symptoms, but couldn't find any.

Because the CEL/P0420 only comes on after consuming more than half the tank of fuel, I'm assuming it's something tank-related. I learned that the Gen 2 Prius doesn't have an EGR valve, so assuming it's something else but fuel-related.
My previous experience with Toyotas with similar problems was fixed with a new gas cap. Maybe I'll try this for the first shot.

Has anyone experienced anything like this?