WimaxHelper::SCHED_TYPE_SIMPLE): UDP packets are received, but delay increments over time (0, 0.5s, 1.0s, 1.5s, ...)

$ ./waf --run "examples/wimax/wimax-simple --duration=4 --scheduler=0"
Waf: Entering directory `/home/arnau/ehas/ns-3-wimax/build'
Waf: Leaving directory `/home/arnau/ehas/ns-3-wimax/build'
'build' finished successfully (0.647s)
1s UdpClient:Send(): TraceDelay TX 1024 bytes to 10.1.1.1 Uid: 1535 Time: 1
1.017s UdpServer:HandleRead(): TraceDelay: RX 1012 bytes from 10.1.1.2 Sequence Number: 0 Uid: 1591 TXtime: 1000000000ns RXtime: 1016999772ns Delay: 16999772ns
1.5s UdpClient:Send(): TraceDelay TX 1024 bytes to 10.1.1.1 Uid: 2203 Time: 1.5
2s UdpClient:Send(): TraceDelay TX 1024 bytes to 10.1.1.1 Uid: 3384 Time: 2
2.02249s UdpServer:HandleRead(): TraceDelay: RX 1012 bytes from 10.1.1.2 Sequence Number: 1 Uid: 3457 TXtime: 1500000000ns RXtime: 2022485725ns Delay: 522485725ns
2.5s UdpClient:Send(): TraceDelay TX 1024 bytes to 10.1.1.1 Uid: 4574 Time: 2.5
3s UdpClient:Send(): TraceDelay TX 1024 bytes to 10.1.1.1 Uid: 5750 Time: 3
3.018s UdpServer:HandleRead(): TraceDelay: RX 1012 bytes from 10.1.1.2 Sequence Number: 2 Uid: 5818 TXtime: 2000000000ns RXtime: 3017999460ns Delay: 1017999460ns
3.5s UdpClient:Send(): TraceDelay TX 1024 bytes to 10.1.1.1 Uid: 6980 Time: 3.5