Why assertion passed but must fail

[bragcustom]

struct data_race_test : rl::test_suite<data_race_test, 2>{
    rl::atomic<int> ff;
    rl::atomic<int> nexec;

    void before(){
        nexec($) = 0;
        ff($) = 0;
    }

    void thread(unsigned id){
        if(id == 0){
            // Thread 0
            if(ff($)==0){       // 1.           7.
                ff($)++;        // 2 ff=1.      8 ff=1.
                nexec($)++;     // 3 exec=1.    9 exec=2.
                assertions1();  // 6.           10 **must fail but doesnt!**
            }else{
                ff($)++;
            }
        }else if(id == 1){
            // Thread 1
            if(nexec($)>0){     // 4.
                ff($)--;        // 5 ff=0.
                if(ff($)==0){
                    nexec($)--;
                    assertions1();
                }
            }
        }
    }

    void assertions1(){
        int ex = nexec($);
        RL_ASSERT(ex >= 0 && ex<=1);
    }
};

int main(){
    rl::test_params p;
    p.search_type = rl::sched_full;
    //p.iteration_count = 1000000;

    rl::simulate<data_race_test>(p);
}

data_race_test iterations: 41 total time: 1 throughput: 41000