xn0 interface "Obytes" not populating

[Jason]

Hi,
  I installed vnstat, which is a bandwidth monitoring app.  I noticed that
for some reason its TX bytes were 0, but the RX bytes were being tracked as
expected.  I don't know how vnstat is collecting its metrics every 5
minutes, but I'm guessing it's similar to how 'netstat -I xn0 -bn' would
collect its details.  After running 'netstat -I xn0 -bn', I notice that
"Obytes" for "<Link#2>" is 0, which may be the problem.  However, the
"Obytes" for the actual IP address seems to be working:

# netstat -I xn0 -bn
Name    Mtu Network       Address              Ipkts Ierrs Idrop
Ibytes    Opkts Oerrs     Obytes  Coll
xn0    1500 <Link#2>      <REDACTED>        22659117     0     0 2579194762
21221967     0          0     0
xn0       - 10.0.10.0/24  10.0.10.40        22567359     -     - 2258113267
21130240     - 3518207522     -

On a different machine that command works fine.  It is running BSD 11 as
well, but it's not using the xn driver -- it uses the fxp driver.  It's
output is:

# netstat -I xn0 -bn
Name    Mtu Network       Address              Ipkts Ierrs Idrop
Ibytes    Opkts Oerrs     Obytes  Coll
em0    1500 <Link#1>      <REDACTED>        1507204306     0     0
1567750969110 100389817     0 80619546746     0
em0       - 192.168.10.0/ 192.168.10.10     1506281038     -     -
1548639235291 100283723     - 79211140098     -

I'm running FreeBSD 11 on both instances, except the one that's having
difficulty is running on AWS, so is on Xen.

Any insight into how to fix this so "Obytes" populates for "Link#2"?

Thanks!