Check the following command to see what actual prefix you’re getting from your ISP:
show log | grep "IA_PD prefix:"
Shows me getting a /56 from my ISP
Jul 27 19:29:00 dhcp6c[19929]: copyin_option: IA_PD prefix: 2406:xxxx:xxxx:c900::/56 pltime=615 vltime=615
Jul 27 19:34:07 dhcp6c[19929]: copyin_option: IA_PD prefix: 2406:xxxx:xxxx:c900::/56 pltime=615 vltime=615
Jul 27 19:39:14 dhcp6c[19929]: copyin_option: IA_PD prefix: 2406:xxxx:xxxx:c900::/56 pltime=615 vltime=615
Jul 27 19:44:21 dhcp6c[19929]: copyin_option: IA_PD prefix: 2406:xxxx:xxxx:c900::/56 pltime=615 vltime=615
Jul 27 19:49:28 dhcp6c[19929]: copyin_option: IA_PD prefix: 2406:xxxx:xxxx:c900::/56 pltime=615 vltime=615
Jul 27 19:54:35 dhcp6c[19929]: copyin_option: IA_PD prefix: 2406:xxxx:xxxx:c900::/56 pltime=615 vltime=615
Jul 27 19:59:42 dhcp6c[19929]: copyin_option: IA_PD prefix: 2406:xxxx:xxxx:c900::/56 pltime=615 vltime=615
Jul 27 20:04:50 dhcp6c[19929]: copyin_option: IA_PD prefix: 2406:xxxx:xxxx:c900::/56 pltime=615 vltime=615
Jul 27 20:09:57 dhcp6c[19929]: copyin_option: IA_PD prefix: 2406:xxxx:xxxx:c900::/56 pltime=615 vltime=615
Jul 27 20:15:04 dhcp6c[19929]: copyin_option: IA_PD prefix: 2406:xxxx:xxxx:c900::/56 pltime=615 vltime=615
Jul 27 20:20:11 dhcp6c[19929]: copyin_option: IA_PD prefix: 2406:xxxx:xxxx:c900::/56 pltime=615 vltime=615
set interfaces ethernet eth0 vif 10 dhcpv6-options pd 0 interface eth1 address '1'
set interfaces ethernet eth0 vif 10 dhcpv6-options pd 0 interface eth1 sla-id '1'
set interfaces ethernet eth0 vif 10 dhcpv6-options pd 0 length '56'
As @gus mentions this only gives me a /64 downstream on my LAN side.