This is how it looks like from an other tunnel end
traceroute to ipv6.l.google.com (2001:4860:0:1001::68) from 2001:470:1f0a:555::2, 30 hops max, 16 byte packets
1 piojan.tunnel.tserv6.fra1.ipv6.he.net (2001:470:1f0a:555::1) 39.597 ms 39.845 ms 43.547 ms
2 gige-g2-4.core1.fra1.he.net (2001:470:0:69::1) 40.628 ms 43.994 ms 57.007 ms
3 10gigabitethernet1-4.core1.ams1.he.net (2001:470:0:47::1) 78.991 ms 71.642 ms 50.803 ms
4 pr61.ams04.net.google.com (2001:7f8:1::a501:5169:1) 358.68 ms !S 319.88 ms !S 378.78 ms !S
From your LG at FRA:
1 14 ms 7 ms 7 ms 2001:470:0:47::1 (10gigabitethernet1-4.core1.ams1.he.net)
2 287 ms 272 ms 272 ms 2001:7f8:1::a501:5169:1 (pr61.ams04.net.google.com)
3 * * * ?
4 284 ms 285 ms 286 ms 2001:4860:0:1001::68
The delay starts from pr61.ams04.net.google.com (2001:7f8:1::a501:5169:1) which is there router at ams-ix.
If we ping the above ipv6 from FRA we have:
round-trip min/avg/max=292/292/292 ms
From London:
round-trip min/avg/max=274/274/274 ms
However from AMS he router:
round-trip min/avg/max=0/0/0 ms
Also this problem don't exist with other providers - for eg:
Hop Node Loss% Sent Last Avg Best Worst StDev ASN Organisation
1. 2001:838:1:1::1 0.0% 5 0.5 0.5 0.4 0.5 0.0 12871 Concepts ICT
ge-1-3-0.breda.ipv6.concepts-ict.net.
2. 2001:838:0:10::1 0.0% 5 2.6 3.1 2.4 5.3 1.3 12871 Concepts ICT
3. 2001:7f8:1::a501:5169:1 0.0% 5 3.4 3.6 3.4 3.8 0.1 AMS-IX-IPV6
pr61.ams04.net.google.com.
4.

100.0 5 0.0 0.0 0.0 0.0 0.0
5. 2001:4860:0:1001::68 0.0% 5 6.8 6.7 6.5 6.8 0.1 15169 Google Inc.
or:
Hop Node Loss% Sent Last Avg Best Worst StDev ASN Organisation
1. 2001:960:800::1 0.0% 5 0.7 0.8 0.7 1.0 0.1 12634 Cybercomm
ge-0-5-800.juniper-1.ipv6.network.scarlet.nl.
2. 2001:7f8:1::a501:5169:1 0.0% 5 1.3 1.3 1.2 1.4 0.1 AMS-IX-IPV6
pr61.ams04.net.google.com.
3. 2001:7f8:1::a501:5169:1 0.0% 5 1.4 1.4 1.2 1.4 0.1 AMS-IX-IPV6
pr61.ams04.net.google.com.
4.

100.0 5 0.0 0.0 0.0 0.0 0.0
5. 2001:4860:0:1001::68 0.0% 5 4.4 4.5 4.1 5.3 0.5 15169 Google Inc.
I am just wondering if there isn't a problem with peering.
You are directing traffic to them through ams but they are not returning it through there.
(something with there bgp or peering).
Edited:
Hop Node Loss% Sent Last Avg Best Worst StDev ASN Organisation
1. 2001:a60:0:200::1 0.0% 5 1.2 1.2 1.0 1.4 0.1 8767
rt4.muc1.m-online.net.
2. 2001:a60::69:0:1:1 0.0% 5 9.8 10.3 9.8 10.6 0.3 8767
rt-decix.m-online.net.
3. 2001:470:0:47::1 0.0% 5 34.0 29.0 16.9 39.0 9.5 Hurricane Electric
10gigabitethernet1-4.core1.ams1.he.net.
4. 2001:7f8:1::a501:5169:1 0.0% 5 21.3 20.1 16.3 29.8 5.9 AMS-IX-IPV6
pr61.ams04.net.google.com.
5.

100.0 5 0.0 0.0 0.0 0.0 0.0
6. 2001:4860:0:1001::68 0.0% 5 23.8 20.3 19.1 23.8 2.0 15169 Google Inc.
This might suggest that there is no problem for the traffic that you are theres transit.
So mayby the problem with google could be narrowed to your prefix only?