ffmpeg frame->pts out-of-order when decode hevc

Question

I try to set pkt->pts to 0,1,2,3,4,5,6, but the frame->pts received is 0,1,4,5,3,6,2 where the pict_type was I, I, B, B, B, B, P.

int num = 0;
pkt->pts = num++;
avcodec_send_packet(dec_ctx, pkt);
avcodec_receive_frame(dec_ctx, frame); 

I tried to read h264 file, it is correct; and I tried to use official 'decode_video.c', it's still incorrect in hevc.

Why the frame->pts is not 0,1,2,3,4,5,6?