Lost bins in coverpoint?

Say now we have a covergroup which is intended to cover the 32-bit range of 'h0 ~ 'hFFFFFFFF.

logic [31:0] test_data;

covergroup test_cg @clk
    option.per_instance = 1;
    v: coverpoint test_data
        iff(reset_n){
        bins v_val_0 ={0};
        bins v_val_n [5] = {[1:'hfffffffe]};
        bins v_val_f = {'hffffffff};
        illegal_bins v_illegal = default;
        }
endgroup

test_cg t_cg = new();
always @(posedge clk)
begin
    test_data = test_data + 'h400000;
    t_cg.sample();
end

initial begin
    test_data = 0;
end

The test_data is increased by 'h400000 at every posedge clk. So the covergroup above is supposed to reach 100% (except v_val_f ) very soon. However, the report give a big surprise as below, i.e. v_val_n[2] and v_val_n[4] are covered while v_val_n[3] is not covered at all. Anyone can help on this issue?

BTW, I'm working on IES-15.22 s024.

Thanks.

test_cg         71.43%, 71.43% (5/7) 269 covergroup test_cg  (clk);
|--v            71.43% (5/7)         271 v: coverpoint test_data
| |--v_val_0    100.00% (19/1)       273 bins v_val_0    = {0};
| |--v_val_n[0] 100.00% (4284/1)     274 bins v_val_n[5] = {[1: 'hfffffffe]};
| |--v_val_n[1] l00.00% (4305/1)     274 bins v_val_n[5] = {[1: 'hfffffffe]};
| |--v_val_n[2] 100.00% (2163/1)     274 bins v_val_n[5] = {[1: 'hfffffffe]};
| |--v_val_n[3] 0.00% (0/1)          274 bins v_val_n[5] = {[1: 'hfffffffe]};
| |--v_val_n[4] 10O.00% (9598/1)     274 bins v_val_n[5] = {[1: 'hfffffffe]};
| |--v_val_f    0.00% (0/1)          275 bins v_val_f    = {'hffffffff};