The output z is just a function of the state of the flip flops.
It can be determined last (after completing the flip flop outputs).
At the last clock transition,
the input is not known and thus J1
is unknown (since Q =1 = 1). Thus, we cannot determine the next value of Q1. But, J2 K2 1 (no matter what x is) and thus we can determine the value for Q2.