Zombies left when backgrounded jobs run for longer than until the main script execs away

The following construct causes a zombie to hang around:

#!/bin/sh
cd /home/tglase/Misc/vpn.w
(sleep 20; sudo route -n add -host x.x.x.x gw 172.28.0.1) &
exec sudo openvpn --config /home/tglase/Misc/vpn.w/vpn-bn-01.ovpn "$@"

Adding an “exec” before the sudo in the subshell fixes this, so perhaps the shell could clean this up itself? (Ideas and patches welcome…)