Asus is really having tough time in getting their firmware to work with different ISP's IPv6 configuration. After long chats with Asus firmware engineer and my ISP's core network team, I came to a conclusion that Asus firmware for IPv6 not getting assigned to WAN of their routers when in PPPOe mode and IPv6 set to Native PPP Stateless.
That ^^ statement may
not always apply as it depends on all of these variables: Ge-Location / ISP / Model of Asus Router / Firmware in use. FWIW My Asus setup is in my forum sig, I'm in Vietnam with a Vietnamese ISP, which like all ISPs, has it's own quirks
My 300 Mbps Up/Down FTTH WAN is PPPOe but, it has a static IPv4 address and a
static IPv6 address (but that's
static within an IPv6 /64 range, which is different than a static / non-changing IPv4 address). The IPv6 connection is exactly as you've described above: Native / PPP / Stateless, which in turn produces a fixed LAN IPv6 /56 range. Everything works perfectly (IPv4 and IPv6 WAN & LAN) and all of those IPv6 tests are passed effortlessly. My own example alone, does mean that the conclusion you've made ^^
IS subject to all of those variables though. The current IPv6 DDNS
issues that are covered in detail elsewhere in this forum / on other threads & that still occur in the latest official stable release from Asus for my router, will hopefully / presumably be fixed in the official RC-3 stable release from Asus and/or the official Merlin stable release for my router. Until then my IPV6 DDNS workaround is what I
posted here - previously, in another thread. AFAIK there's no RC3-3 Beta for the Asus router that I am using anyway - and/or I've missed it so far
but I'll be waiting for stable releases anyway.
Edit: Enthusiasm got the better of me, so I changed the firmware to Merlin's 386.5 Alpha 2
advance release. Works perfectly for me so far!