iperf3 Transfer Summary shows NULL above certain point

I was able to confirm/validate the failure on a set of LXC containers:

bladernr@xenial:~$ iperf3 -c 10.148.80.151 -i 60 -n 999G
Connecting to host 10.148.80.151, port 5201
[ 4] local 10.148.80.13 port 44408 connected to 10.148.80.151 port 5201
[ ID] Interval Transfer Bandwidth Retr Cwnd
[ 4] 0.00-60.00 sec 464 GBytes 66.4 Gbits/sec 236 3.00 MBytes
[ 4] 60.00-120.00 sec 456 GBytes 65.3 Gbits/sec 0 3.00 MBytes
[ 4] 120.00-130.09 sec 78.8 GBytes 67.1 Gbits/sec 0 3.00 MBytes
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval Transfer Bandwidth Retr
[ 4] 0.00-130.09 sec 999 GBytes 66.0 Gbits/sec 236 sender
[ 4] 0.00-130.09 sec 999 GBytes 66.0 Gbits/sec receiver

bladernr@xenial:~$ iperf3 -c 10.148.80.151 -i 60 -n 1500G
Connecting to host 10.148.80.151, port 5201
[ 4] local 10.148.80.13 port 44538 connected to 10.148.80.151 port 5201
[ ID] Interval Transfer Bandwidth Retr Cwnd
[ 4] 0.00-60.00 sec 456 GBytes 65.3 Gbits/sec 2851 1.16 MBytes
[ 4] 60.00-120.00 sec 469 GBytes 67.1 Gbits/sec 66 2.48 MBytes
q[ 4] 120.00-180.00 sec 469 GBytes 67.2 Gbits/sec 137 3.02 MBytes
[ 4] 180.00-193.34 sec 106 GBytes 68.2 Gbits/sec 0 3.02 MBytes
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval Transfer Bandwidth Retr
[ 4] 0.00-193.34 sec 0.00 (null)s 66.6 Gbits/sec 3054 sender
[ 4] 0.00-193.34 sec 0.00 (null)s 66.6 Gbits/sec receiver

bladernr@xenial:~$ apt-cache policy iperf3
iperf3:
  Installed: 3.0.11-1
  Candidate: 3.0.11-1
  Version table:
 *** 3.0.11-1 500
        500 http://archive.ubuntu.com/ubuntu xenial/universe amd64 Packages
        100 /var/lib/dpkg/status

This also affects bionic:
bladernr@xenial:/etc/apt/preferences.d$ iperf3 -c 10.148.80.151 -i 60 -n 1500G
Connecting to host 10.148.80.151, port 5201
[ 4] local 10.148.80.13 port 44836 connected to 10.148.80.151 port 5201
[ ID] Interval Transfer Bandwidth Retr Cwnd
[ 4] 0.00-60.00 sec 457 GBytes 65.4 Gbits/sec 2546 1.77 MBytes
[ 4] 60.00-120.00 sec 482 GBytes 69.1 Gbits/sec 89 3.04 MBytes
[ 4] 120.00-180.00 sec 478 GBytes 68.4 Gbits/sec 25 3.01 MBytes
[ 4] 180.00-193.22 sec 82.7 GBytes 53.7 Gbits/sec 1091 1.60 MBytes
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval Transfer Bandwidth Retr
[ 4] 0.00-193.22 sec 0.00 (null)s 66.7 Gbits/sec 3751 sender
[ 4] 0.00-193.22 sec 0.00 (null)s 66.7 Gbits/sec receiver

iperf Done.

bladernr@xenial:/etc/apt/preferences.d$ apt-cache policy iperf3
iperf3:
  Installed: 3.1.3-1
  Candidate: 3.1.3-1
  Version table:
 *** 3.1.3-1 500
        500 http://archive.ubuntu.com/ubuntu bionic/universe amd64 Packages
        100 /var/lib/dpkg/status
     3.0.11-1 -10
        -10 http://archive.ubuntu.com/ubuntu xenial/universe amd64 Packages
bladernr@xenial:/etc/apt/preferences.d$ apt-cache policy libiperf0
libiperf0:
  Installed: 3.1.3-1
  Candidate: 3.1.3-1
  Version table:
 *** 3.1.3-1 500
        500 http://archive.ubuntu.com/ubuntu bionic/universe amd64 Packages
        100 /var/lib/dpkg/status
     3.0.11-1 -10
        -10 http://archive.ubuntu.com/ubuntu xenial/universe amd64 Packages

This is resolved in Cosmic:
bladernr@cosmic:~$ iperf3 -c 10.148.80.151 -n 1500G -i 60
Connecting to host 10.148.80.151, port 5201
[ 5] local 10.148.80.5 port 58834 connected to 10.148.80.151 port 5201
[ ID] Interval Transfer Bitrate Retr Cwnd
[ 5] 0.00-60.00 sec 447 GBytes 64.0 Gbits/sec 845 1.04 MBytes
[ 5] 60.00-120.00 sec 435 GBytes 62.3 Gbits/sec 653 898 KBytes
[ 5] 120.00-180.00 sec 444 GBytes 63.5 Gbits/sec 47 3.01 MBytes
[ 5] 180.00-203.80 sec 174 GBytes 62.8 Gbits/sec 0 3.01 MBytes
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval Transfer Bitrate Retr
[ 5] 0.00-203.80 sec 1.46 TBytes 63.2 Gbits/sec 1545 sender
[ 5] 0.00-203.80 sec 1.46 TBytes 63.2 Gbits/sec receiver

iperf Done.
bladernr@cosmic:~$ apt-cache policy iperf3
iperf3:
  Installed: 3.6-2
  Candidate: 3.6-2
  Version table:
 *** 3.6-2 500
        500 http://archive.ubuntu.com/ubuntu cosmic/universe amd64 Packages
        100 /var/lib/dpkg/status