CentOS 7 IPv6 not reachable

Hello - at the london data center I have a CentOS 7 linode running as a NSD slave (nameserver). I can reach it via IPv4 but I can not reach it via IPv6 which unfortunately means that it is not able to get DNS updates. Use to work just fine.

IPv6 address is 2a01:7e00::f03c:91ff:fe18:8a7e

Output of ifconfig command:

eth0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        inet 178.79.185.217  netmask 255.255.255.0  broadcast 178.79.185.255
        inet6 2a01:7e00::f03c:91ff:fe18:8a7e  prefixlen 64  scopeid 0x0<global>
        inet6 fe80::f03c:91ff:fe18:8a7e  prefixlen 64  scopeid 0x20<link>
        ether f2:3c:91:18:8a:7e  txqueuelen 1000  (Ethernet)
        RX packets 74732  bytes 30659029 (29.2 MiB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 72874  bytes 12423019 (11.8 MiB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

configuration of /etc/sysconfig/network-scripts/ifcfg-Wired_connection_1:

HWADDR=F2:3C:91:18:8A:7E
TYPE=Ethernet
BOOTPROTO=dhcp
DEFROUTE=yes
IPV4_FAILURE_FATAL=no
IPV6INIT=yes
IPV6_AUTOCONF=yes
IPV6_DEFROUTE=yes
IPV6_FAILURE_FATAL=no
NAME="Wired connection 1"
UUID=21f2e362-f81c-3cc6-9b33-ae629bba92f8
ONBOOT=yes
AUTOCONNECT_PRIORITY=-999
PEERDNS=yes
PEERROUTES=yes
IPV6_PEERDNS=yes
IPV6_PEERROUTES=yes

In the past once I had an issue on a different linode where a change in CentOS 7 caused it to be using a randomized address within the assigned subnet rather than a static address, not sure if that is the issue here or not?

My other CentOS 7 NSD slave and my NSD master don't have this issue, their IPv6 is reachable.

Any help would be appreciated, manually copying updated zone files over gets very old very quickly ;)