I’m running a FreeBSD-12.1 image, updated using freebsd-update to FreeBSD-13.0-RELEASE.

When I enabled ipv6 on the droplet, the service /usr/local/etc/rc.d/digitalocean was being run on startup which wiped the ipv4 and ipv6 in /etc/rc.conf every boot. To solve your problem, there are 2x steps you’ll need to do:

1) configure ipv4/ipv6 addresses, defaultroute, and if desired, aliases.
(you can get all of the network info from the network tab of your droplet in the UI)

# /etc/rc.conf

# default routes
defaultrouter="111.222.111.1"
ipv6_defaultrouter="2607:f0d0:1002:51::1"

# assign ip addresses
ifconfig_vtnet0="inet  111.222.111.33 netmask 255.255.240.0"
ifconfig_vtnet0_ipv6="inet6 2607:f0d0:1002:51::a001 prefixlen 64"
ipv6_activate_all_interfaces="yes"

test this before continuing:

sudo service netif restart
sudo service routing restart

2) disable the digitalocean service to prevent your config from being wiped every reboot.

# /etc/rc.conf

# digitalocean="YES"

# for me, disabling the service in rc.conf had no effect,
# so I renamed the service
mv /usr/local/etc/rc.d/digitalocean /usr/local/etc/rc.d/digitalocean.orig