Edit: I first thought it was a routing issue on my server, now I'm thinking it has to do with the RouterAdvertisements sent from the server.

Edit:

tcpdump shows my ping request entering the server in interface iot:

#tcpdump -eni iot ip6 15:49:12.290030 c0:a5:e8:46:79:31 > 56:ce:59:c6:b6:2d, ethertype IPv6 (0x86dd), length 118: 2a02:a46e:53ea:c001:dc81:6cfc:2115:899 > 2a00:1450:400e:80f::200e: ICMP6, echo request, id 30134, seq 1, length 64 

but not being forwarded to eth0 (only the same packet WITH vlan tag):

#tcpdump -eni eth0 ip6 15:50:55.083174 c0:a5:e8:46:79:31 > 56:ce:59:c6:b6:2d, ethertype 802.1Q (0x8100), length 122: vlan 2, p 0, ethertype IPv6 (0x86dd), 2a02:a46e:53ea:c001:dc81:6cfc:2115:899 > 2a00:1450:400e:80f::200e: ICMP6, echo request, id 30653, seq 1, length 64 

Pinging from the iot-vlan interface:

# ping6 2a00:1450:400e:80f::200e -I iot PING 2a00:1450:400e:80f::200e(2a00:1450:400e:80f::200e) from 2a02:a46e:53ea:c001:54ce:59ff:fec6:b62d iot: 56 data bytes From 2a02:a46e:53ea:c001:54ce:59ff:fec6:b62d icmp_seq=1 Destination unreachable: Address unreachable 

Pinging from a device in the iot-vlan also does not work.

# ping6 2a00:1450:400e:80f::200e -I iot PING 2a00:1450:400e:80f::200e(2a00:1450:400e:80f::200e) from 2a02:a46e:53ea:c001:54ce:59ff:fec6:b62d iot: 56 data bytes From 2a02:a46e:53ea:c001:54ce:59ff:fec6:b62d icmp_seq=1 Destination unreachable: Address unreachable 

edit: Pinging from the address of the iot interface works (tnx @u1686_grawity)

$ ping6 google.com -I "2a02:a46e:53ea:c000:54ce:59ff:fec6:b62d" PING google.com(ams17s12-in-x0e.1e100.net (2a00:1450:400e:810::200e)) from 2a02:a46e:53ea:c000:54ce:59ff:fec6:b62d : 56 data bytes 64 bytes from ams17s12-in-x0e.1e100.net (2a00:1450:400e:810::200e): icmp_seq=1 ttl=60 time=7.64 ms 

Pinging from a device in the iot-vlan does not work.

$ ping6 google.com PING google.com (2a00:1450:400e:803::200e) 56 data bytes --- google.com ping statistics --- 236 packets transmitted, 0 received, 100% packet loss, time 240617ms 

ip address of device on iot-vlan:

$ ip -6 addr show dev wlp1s0 3: wlp1s0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default qlen 1000 inet6 2a02:a46e:53ea:c001:dc81:6cfc:2115:899/64 scope global temporary dynamic    valid_lft 3576sec preferred_lft 1776sec inet6 2a02:a46e:53ea:c001:4f32:ea73:f336:3907/64 scope global dynamic mngtmpaddr noprefixroute valid_lft 3576sec preferred_lft 1776sec inet6 fe80::aff0:9c03:3ce:7601/64 scope link noprefixroute   valid_lft forever preferred_lft forever 

route of device on iot-vlan

$ ip -6 route 2a02:a46e:53ea:c001::/64 dev wlp1s0 proto ra metric 600 pref medium fe80::/64 dev veth0ad8518 proto kernel metric 256 pref medium fe80::/64 dev docker0 proto kernel metric 256 pref medium fe80::/64 dev veth998c5e2 proto kernel metric 256 pref medium fe80::/64 dev wlp1s0 proto kernel metric 1024 pref medium default via fe80::54ce:59ff:fec6:b62d dev wlp1s0 proto ra metric 20600 pref medium 

Edit: I first thought it was a routing issue on my server, now I'm thinking it has to do with the RouterAdvertisements sent from the server.