This is a bug in ifupdown, not isc-dhcp-client. It's fixed in 0.8.11 by the change to ifupdown's wait-for-ll6.sh:
--- ifupdown-0.8.10/wait-for-ll6.sh 2015-12-01 16:50:26.000000000 -0500
+++ ifupdown-0.8.11/wait-for-ll6.sh 2016-04-20 08:57:37.000000000 -0400
@@ -4,7 +4,7 @@
delay=${IF_LL_INTERVAL:-0.1}
for attempt in $(seq 1 $attempts); do
- lladdress=$(ip -6 -o a s dev "$IFACE" scope link)
+ lladdress=$(ip -6 -o a s dev "$IFACE" scope link -tentative)
if [ -n "$lladdress" ]; then
attempt=0
break
