rtsold will not run on VLAN interfaces
A VLAN interface for a WAN will not pull an IPv6 address via DHCPv6 with a default configuration.
In a default configuration, dhcp6c is launched via rtsold. When a VLAN-based interface is assigned, rtsold fails to run which means dhcp6c is never launched.
Running rtsold in debug mode produces the following log message:
Mar 20 12:41:41 pfsense rtsold: <make_packet> link-layer address option has null length on cpsw0_vlan10. Treat as not included.
The same message is not logged when the parent interface is used directly.
If I check "Do not wait for a RA" on the WAN settings which bypasses the need for rtsold, dhcp6c is launched directly and pulls an address.