Is routing with 2 stacked OPNsense routers broken?

[gogolathome]

I have router A connected to my dual stacked bridged cable modem.
Router B is connected to Hyper-V and is behind router A. Connected to router B is a virtual Ubuntu linux host.

Internet -- Router A -- Router B -- Linux host

I configured everything to my best knowledge and every machine is dual stacked with IPv4 and IPv6. I get a /56 prefix from my ISP and delegated a /62 to router B. This just a test setup.

Now here comes the problem, there is no IPv6 connection possible:

Ping6 to 2a00:1450:400e:80e::200e from Linux host behind router B give no reply. I see packets leaving on router B WAN interface and coming in on Router A LAN interface. But they don't leave Router A WAN!

Ping6 to 2a00:1450:400e:80e::200e from Router B leave WAN interface from router A and get a reply, but they don't leave LAN interface from Router A to WAN interface from Router B. End result is no reply.

It seems that packets get lost on Router A and I am breaking my head why.
The routing table on Router A seems ok, but I have read an old topic about some problems with downstream routers: https://forum.opnsense.org/index.php?topic=7719.0