Metric doesn't go into effect for DHCPv6 proto

I have a primary WAN and a 5G connection that I use for failover. My config looks like this:

config interface 'wan6'
        option device '@wan'
        option metric '10'
        option reqaddress 'try'
        option reqprefix 'auto'
        option proto 'dhcpv6'
        option sourcefilter '0'

config interface 'wan'
        option proto 'pppoe'
        option device 'br-wan'
        option enabled '0'
        option username 'USERNAME'
        option password 'PASSWORD'
        option ipv6 '0'
        option metric '10

config interface 'one_w_addr' # I do this because my wan6 interface does not get an IPv6 address only an IPv6-PD
        option device '@wan'
        option proto 'static'
        list ip6addr 'xxxx::1234'
        option ip6gw 'xxxx:1144'
        option metric '10'
        option delegate '0'

config interface '4G'
        option proto 'dhcp'
        option device 'usb0'
        option metric '50'

config interface '4G_6'
        option proto 'dhcpv6'
        option device '@4G'
        option reqaddress 'try'
        option reqprefix 'auto'
        option metric '50'
        option sourcefilter '0'

For IPv4 routes everything is great:

root@openwrt:~# route -n
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
0.0.0.0         x.x.x.x         0.0.0.0         UG    10     0        0 pppoe-wan
0.0.0.0         10.11.208.1     0.0.0.0         UG    50     0        0 usb0
10.11.208.0     0.0.0.0         255.255.255.0   U     50     0        0 usb0
x.x.x.x         x.x.x.x         255.255.255.255 UGH   10     0        0 pppoe-wan
x.x.x.x         0.0.0.0         255.255.255.255 UH    0      0        0 pppoe-wan
x.x.x.x         x.x.x.x         255.255.255.255 UGH   10     0        0 pppoe-wan
172.16.42.0     0.0.0.0         255.255.255.0   U     0      0        0 g-br-lan
192.168.16.0    0.0.0.0         255.255.255.0   U     0      0        0 br-lan

For IPv6 on the other hand:

root@alchemilla-difuse:~# route -A inet6 | grep "pppoe-wan"
xxxx::1234/128                              ::                                      U     10     1        0 pppoe-wan # Static interface Address!
fe80::75a6:bad2:38f:ff4e/128                ::                                      U     256    1        0 pppoe-wan
fe80::f6bf:a8ff:fe75:1144/128               ::                                      U     256    3        0 pppoe-wan
::/0                                        fe80::f6bf:a8ff:fe75:1144               UG    10     5        0 pppoe-wan
xxxx::1234/128                              ::                                      Un    0      9        0 pppoe-wan
fe80::75a6:bad2:38f:ff4e/128                ::                                      Un    0      5        0 pppoe-wan


root@alchemilla-difuse:~# route -A inet6 | grep "usb0"
2402:3a80:1cbb:6240::/64                    ::                                      U     256    3        0 usb0
fe80::/64                                   ::                                      U     256    1        0 usb0    
::/0                                        fe80::606e:60ff:fefd:5391               UG    512    1        0 usb0 # Why is this 512?    
2402:3a80:1cbb:6240::/128                   ::                                      Un    0      3        0 usb0    
2402:3a80:1cbb:6240:843f:ceff:fee8:47b7/128 ::                                      Un    0      3        0 usb0    
fe80::/128                                  ::                                      Un    0      3        0 usb0    
fe80::843f:ceff:fee8:47b7/128               ::                                      Un    0      2        0 usb0    
ff00::/8                                    ::                                      U     256    5        0 usb0   

Notice how usb0 has 512 as metric? At startup almost always the 5G interface comes up first this means for some reason it gets a lower metric. This would mean all my traffic start going through the WWAN interface which is not desirable..

I found similar issues namely this one but haven't seen any responses there either for the same, it would be nice to know why this happens and if there's a remedy for it.