关于北京联通 IPv6 Delegation 设置

我的网络拓扑：

pc -> 软路由 -> 光猫(非桥接)

我可以从光猫上看到联通给下发了网络前缀：

2408:8207:xxxx:xxxx::/60

但是我的软路由通过 dhcpv6 client 可以拿到 ipv6 地址，但是拿不到 ipv6 prefix

ifstatus wan6
{
	"up": true,
	"pending": false,
	"available": true,
	"autostart": true,
	"dynamic": false,
	"uptime": 66334,
	"l3_device": "eth0",
	"proto": "dhcpv6",
	"device": "eth0",
	"metric": 0,
	"dns_metric": 0,
	"delegation": true,
	"ipv4-address": [

	],
	"ipv6-address": [
		{
			"address": "2408:8207:24c7:57c0:xxx:xxxx:xxxx",
			"mask": 64,
			"preferred": 136922,
			"valid": 223322
		}
	],
	"ipv6-prefix": [

	],
	"ipv6-prefix-assignment": [

	],
	"route": [
		{
			"target": "2408:8207:24c7:57c0::",
			"mask": 64,
			"nexthop": "::",
			"metric": 256,
			"valid": 223322,
			"source": "::/0"
		},
		{
			"target": "::",
			"mask": 0,
			"nexthop": "fe80::1",
			"metric": 512,
			"valid": 1709,
			"source": "2408:8207:24c7:57c0:dea6:xxx:xxxx:xxxx/64"
		}
	],
	"dns-server": [
		"2408:8207:24c7:57c0:a23e:6bff:fea0:a2a7",
		"fe80::a23e:6bff:fea0:a2a7",
		"2408:8000:1010:1::8",
		"2408:8000:1010:2::8"
	],
	"dns-search": [

	],
	"neighbors": [

	],
	"inactive": {
		"ipv4-address": [

		],
		"ipv6-address": [

		],
		"route": [

		],
		"dns-server": [

		],
		"dns-search": [

		],
		"neighbors": [

		]
	},
	"data": {
		"passthru": "001700102408820724c757c0a23ea0a2a7"
	}
}

初步猜测是光猫的 dhcpv6 不支持下发 ipv6 delegation，或者我的 wan6 接口需要一些特殊配置

有熟悉的兄弟望指点，谢谢！