Same problem here.
My ISP is Deutsche Telekom which uses PPPoE. The optimal MTU on the ISP interface is 1492 bytes (1500 - 8 Bytes PPPoE overhead).
Both my tunnel on the website and the tunnel interface in pfSense are set to 1472 (1492 - 20 Bytes 6in4 overhead).
Pinging kb.vmware.com with disabled fragmentation works fine (1472 - 48 bytes ICMPv6 overhead = 1424 bytes payload):
ping6 -M do -s 1424 -c 3 kb.vmware.com
PING kb.vmware.com(2a02:e980:46::13) 1424 data bytes
1432 bytes from 2a02:e980:46::13: icmp_seq=1 ttl=58 time=35.7 ms
1432 bytes from 2a02:e980:46::13: icmp_seq=2 ttl=58 time=36.3 ms
1432 bytes from 2a02:e980:46::13: icmp_seq=3 ttl=58 time=35.5 ms
--- kb.vmware.com ping statistics ---
3 packets transmitted, 3 received, 0% packet loss, time 2002ms
rtt min/avg/max/mdev = 35.591/35.908/36.388/0.378 ms
All IPv6-enabled hosts in the network are affected. Still, all other IPv6 sites except kb.vmware.com and blogs.vmware.com work fine.
Trying to set the tunnel MTU to a lower or higher value didn't help either.