alt Hacker NewsI don't know enough to speak about that particular domain, but if the junior is writing something the senior can't understand, that's always going to be a problem. That code becomes the team's responsibility, and that code needs to be able to be maintained by the entire team, not only by the junior with something to prove.
Who is getting called at 2 AM when something breaks? Not the junior.
Hehe you think L9s do oncall? Senior not understanding is a problem all right. It's not necessarily junior's problem.