Iperf3 high retr count, what causes it?

I'm running a fresh build of snapshot (r14692-0830ae3a2f) on a Netgear R7800
Linux kernel 5.4.70

Running iperf3 in server mode on the router.
and iperf3 in client mode on a 5Ghz Wifi connected Ubuntu system, I see high retr counts when running with the iperf3 client in Reverse mode (in other words when the client is receiving rather than sending). And the transfer rate is 50% lower.

$ iperf3 -c 192.168.1.1 -i 10 -t 30 

Connecting to host 192.168.1.1, port 5201
[  4] local 192.168.80.151 port 58110 connected to 192.168.1.1 port 5201
[ ID] Interval           Transfer     Bandwidth       Retr  Cwnd
[  4]   0.00-10.00  sec   594 MBytes   498 Mbits/sec    0   65.6 KBytes       
[  4]  10.00-20.00  sec   616 MBytes   517 Mbits/sec    0   65.6 KBytes       
[  4]  20.00-30.00  sec   650 MBytes   545 Mbits/sec    0   65.6 KBytes       
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval           Transfer     Bandwidth       Retr
[  4]   0.00-30.00  sec  1.82 GBytes   520 Mbits/sec    0             sender
[  4]   0.00-30.00  sec  1.82 GBytes   520 Mbits/sec                  receiver

iperf Done.

$ iperf3 -c 192.168.1.1 -i 10 -t 30 -R

Connecting to host 192.168.1.1, port 5201
Reverse mode, remote host 192.168.1.1 is sending
[  4] local 192.168.80.151 port 58114 connected to 192.168.1.1 port 5201
[ ID] Interval           Transfer     Bandwidth
[  4]   0.00-10.00  sec   291 MBytes   244 Mbits/sec                  
[  4]  10.00-20.00  sec   242 MBytes   203 Mbits/sec                  
[  4]  20.00-30.00  sec   153 MBytes   128 Mbits/sec                  
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval           Transfer     Bandwidth       Retr
[  4]   0.00-30.00  sec   690 MBytes   193 Mbits/sec  471             sender
[  4]   0.00-30.00  sec   687 MBytes   192 Mbits/sec                  receiver

iperf Done.

This seems to be new behavior in the latest snapshot code.
I don't remember this being the case in previous snapshot code.

What is causing this disparity in send vs receive throughput?
And what is causing the apparent high retr count?