If you have a dual stack network you can have different MTU settings for ipv4 and ipv6.
Even though the adapter itself shows only one MTU, for ipv6 connections, the v6-MTU can be different.
As the hurricane electric tunnel has a maximum MTU of 1480, one needs to lower the interface MTU to a reasonable value.
If you have a pfsense firewall, simply set the LAN interface MTU to the value you would like to announce via radvd.
This leads to the following network/path-mtu for a linux system:
2: enp0s25: mtu 1500 qdisc fq_codel state UP group default qlen 1000
link/ether 3c:97:0e:eb:a1:c4 brd ff:ff:ff:ff:ff:ff
But thats only the IPv4 MTU. For ipv6 we see:
$ cat /proc/sys/net/ipv6/conf/enp0s25/mtu