The problem with IPv6 is that starlink changed their ethernet MAC handling code back in mid April 2024… with the software release 6fd45cff-4f03-4efc-8c24-26caaac9663c.uterm_manifest.release.
The newer software only wants to support a single client MAC address on the ethernet cable. the standard way that peplink routers handle ipv6 is to allow the ethernet MAC addresses pass through from the LAN to the WAN network. Triggering the packet loss… you will see that when IPv6 is working IPv4 stops…and vice versa.
From a support ticket:
"Having more than 1 L3 devices behind the UT is currently unsupported, I apologize for any inconvenience. We put some measures in place to work around this for you, but I cannot guarantee this will work indefinitely. "
Now, I would not run a Peplink system in ipv6 passthrough since you receive no ipv6 security from peplink. I use a PFSense router to handle the v6 traffic, and peplink for the v4.
That workaround is still in place on my account, so I still have three MAC addresses on my starlink WAN side and it has worked since, you might be able to request a similar update.
I also have a V2 starlink that was failing in bypass mode and after some deep debugging with a network switch span/mirror we found that DHCP packets were leaking from one of the WLAN Vlans onto the WAN, causing a second MAC address to be seen by starlink’s terminal and triggering this new behavior as well.