After upgrading to Sagitta 1.4.3, the router no longer obtains a default route for IPv6 from the ISP, however IPv4 works fine. Downgrading to 1.4.2 LTS fixed the issue.
On 1.4.3 LTS, I had to manually add a static route with the command `set protocols static route6 ::/0 next-hop fe80::201:5cff:fe81:c046 interface eth0`. This temporarily solves the issue, but these link-local addresses can change.
After downgrading to 1.4.2, the static route is no longer needed as it automatically adds a kernel route learned from the ISP.
EDIT - Almost forgot, here are the relevant configs:
```
admin@router:~$ sh configuration commands | strip-private | grep "eth0"
set firewall group interface-group WAN_IG interface 'eth0'
set interfaces ethernet eth0 address 'dhcp'
set interfaces ethernet eth0 address 'dhcpv6'
set interfaces ethernet eth0 description 'WAN'
set interfaces ethernet eth0 dhcpv6-options pd 0 interface eth1.4 address '1'
set interfaces ethernet eth0 dhcpv6-options pd 0 interface eth1.4 sla-id '0'
set interfaces ethernet eth0 dhcpv6-options pd 0 interface eth1.5 address '1'
set interfaces ethernet eth0 dhcpv6-options pd 0 interface eth1.5 sla-id '1'
set interfaces ethernet eth0 dhcpv6-options pd 0 length '56'
set interfaces ethernet eth0 dhcpv6-options rapid-commit
set interfaces ethernet eth0 hw-id 'xx:xx:xx:xx:xx:f7'
set interfaces ethernet eth0 offload gro
set interfaces ethernet eth0 offload gso
set interfaces ethernet eth0 offload rfs
set interfaces ethernet eth0 offload rps
set interfaces ethernet eth0 offload sg
set interfaces ethernet eth0 offload tso
set interfaces ethernet eth0 ring-buffer rx '4096'
set interfaces ethernet eth0 ring-buffer tx '4096'
set nat source rule 1 outbound-interface name 'eth0'
```
If you need anything else, let me know