From the container I see, that it doesn’t go beyond the bridge:
# traceroute6 -w1 -n 2001:4860:4860::8888
traceroute to 2001:4860:4860::8888 (2001:4860:4860::8888), 30 hops max, 80 byte packets
1 2000:SOMEIPV6::2 0.176 ms 0.013 ms 0.011 ms
2 * * *
3 * * *
4 * * *
5 * * *
6 * * *
7 * * *
8 * * *
9 * * *
10 * * *
11 * * *
12 * * *
13 * * *
14 * * *
15 * * *
16 * * *
17 * * *
18 * * *
19 * * *
20 * * *
21 * * *
22 * * *
23 * * *
24 * * *
25 * * *
26 * * *
27 * * *
28 * * *
29 * * *
30 * * *
From the host I see:
# traceroute6 -w1 -n 2000:SOMEIPV6:216:3eff:fe5a:68fe
traceroute to 2000:SOMEIPV6:216:3eff:fe5a:68fe (2000:SOMEIPV6:216:3eff:fe5a:68fe), 30 hops max, 72 byte packets
1 2000:SOMEIPV6:216:3eff:fe5a:68fe 0.010 ms 0.005 ms 0.005 ms
Unfortunately I cannot go further upstream as my cloud provider will not let me access that.
My cloud provider routes traffic to the MAC address of my VMs NIC and NDP was switched off in sysctl.conf.
I also have a very similar setup from a few years ago working perfectly with Ubuntu + LXD. It is running now. Back then @tomp helped me out greatly and I was able to contribute a tutorial about it: Getting universally routable IPv6 Addresses for your Linux Containers on Ubuntu 18.04 with LXD 4.0 on a VPS
I must be missing something that will make me want to hit myself once found out. 