Install this script as /etc/NetworkManager/dispatcher.d/12-getv6addr as a hack to get NM to acquire DHCPv6 stateful addresses at boot time

12-getv6addr

#!/bin/bash
set -x

DEVICE=$1
ACTION=$2

#if [[ $DEVICE = "eth1" &&  $ACTION = "up" ]] ; then
if [[ $ACTION = "up" ]]; then
    MYNAME=$(echo $0 | awk -F '/' '{print $NF}')
    WORKDIR=$(mktemp -d /var/tmp/${MYNAME}XXXX)
    LEASEFILE=${WORKDIR}/dhcp6-${DEVICE}
    PIDFILE=${WORKDIR}/dhclient6.pid

    cp /sbin/dhclient $WORKDIR
    ${WORKDIR}/dhclient -6 -1 -lf $LEASEFILE -pf $PIDFILE $DEVICE -sf /bin/true
    kill $(cat $PIDFILE) || true
    ADDR=$(awk '/iaaddr/ {print $2}' $LEASEFILE)
    if [ -n "$ADDR" ]; then
      ip addr add $ADDR/64 dev $DEVICE || true
    fi
    rm -rf $WORKDIR
fi

exit 0