Processes returns wrong results on OpenMPI 4.0.4 unless $OMPI_MCA_pml is set to "^ucx"

gistfile1.txt

int main(int argc, char **argv)
{
        int mpirank, mpisize;
        int tabsize = atoi(*(argv + 1));

        MPI_Init(&argc, &argv);
        MPI_Comm_rank(MPI_COMM_WORLD, &mpirank);
        MPI_Comm_size(MPI_COMM_WORLD, &mpisize);

        unsigned long int sum = 0;
        int rcvsize = tabsize / mpisize;
        int *rcvbuf = malloc(rcvsize * sizeof(int));
        int *tab = malloc(tabsize * sizeof(int));
        int totalsum = 0;

        if(mpirank == 0){

                for(int i=0; i < tabsize; i++){
                        *(tab + i) = 1;
                }


        }
                MPI_Scatter(tab, tabsize/mpisize, MPI_INT, rcvbuf, tabsize/mpisize, MPI_INT, 0, MPI_COMM_WORLD);
   
        for(int i=0; i < tabsize/mpisize; i++){
                sum += *(rcvbuf + i);
        }

        printf("%d sum = %ld %d\n", mpirank, sum, tabsize/mpisize);
        MPI_Reduce(&sum, &totalsum, 1, MPI_INT, MPI_SUM, 0, MPI_COMM_WORLD);

        if(mpirank == 0){
                printf("The totalsum = %li\n", totalsum);
        }

        MPI_Finalize();

        return 0;
}