Little Stinker!! lol. We had the same set up with a barrier across the bottom of the stairs and Angel did the same thing.
But we changed it, in fact, I just took a picture of it today to post! So we now use a wire linen shelf, 12 inches tall and about as wide as a stair. We put it across the third stair. We use the third one because he won't jump through anything higher - lol! However, he can jump over it as it is now. He just doesn't! And that is fine because we are getting older and don't need to be tripping on stuff on the stairs!
The WILL find a way to do what they want to!