netstart: adding a vlan to a bridge fails to set parent interface of vlan to promisc?

I haven't been able to quite get my finger on the sequence of events that cause netstart to leave the parent interfaces of my vlans in non-promisc, but I was curious if anyone could provide any guidance as to what's going on. Rerunning netstart after boot fixes the problem – I've attached a partial diff of `ifconfig` before and after rerunning netstart to show what happens.

diff --git a/before-novmd b/after-novmd
index 573c2aa..0fe8fad 100644
--- a/before-novmd
+++ b/after-novmd
@@ -4,7 +4,7 @@ lo0: flags=8049<UP,LOOPBACK,RUNNING,MULTICAST> mtu 32768
inet6 ::1 prefixlen 128
inet6 fe80::1%lo0 prefixlen 64 scopeid 0x8
inet 127.0.0.1 netmask 0xff000000
-em0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> mtu 1500
+em0: flags=8b43<UP,BROADCAST,RUNNING,PROMISC,ALLMULTI,SIMPLEX,MULTICAST> mtu 1500
lladdr 40:62:31:01:4e:18
description: nycmesh-lbe-1659
index 1 priority 0 llprio 3

hostname.em0:
description "nycmesh-lbe-1659"
group trunk
up

hostname.vlan2:
description "nycmesh-lbe-1659 WAN VLAN"
group nycmesh
group bridged
parent em0 vnetid 3
up

hostname.bridge2:
description "Bridged WAN"
group wan
group bridge
add vether2
add vlan2
up

NB: there is a second bridge/vlan combo on em0 that I've omitted for brevity.