interfaces wwan <ifname> address dhcp, address dhcpv6, and ipv6 address autoconf assume the network answers a real DHCP(v4)/DHCPv6 exchange, or that the kernel receives real Router Advertisements. Modern QMI raw-ip modems don't work that way, the address, gateway, and DNS are negotiated once at connect time and exposed by ModemManager as a "static" bearer config (mmcli -b <n>), not handed out over an actual DHCP/RA exchange on the link.

Confirmed live against a real Sierra MC7455 (Google Fi/T-Mobile, APN h2g2):

IPv4 (address dhcp) happens to mostly work today, but only by accident, the bearer's gateway answers a plain DHCPv4 request in milliseconds, almost certainly the modem's firmware synthesizing a reply locally rather than anything reaching the carrier network.
IPv6 (address dhcpv6) never gets a response on port 547 at all, unicast or multicast, the client hangs forever while the interface looks "configured" with no address.
IPv6 via SLAAC (ipv6 address autoconf) is a silent dead end on this hardware, confirmed via a full tcpdump capture that zero ICMPv6 RS/RA traffic of any kind crosses the interface. This modem resolves its IPv6 address entirely over the air on its own and only reports the finished result to ModemManager, so the kernel's accept_ra/autoconf sysctls have nothing to ever autoconfigure from.

Looking at ModemManager's documentation - https://modemmanager.org/docs/modemmanager/ip-connectivity-setup-in-lte-modems/ it says

QMI modems running in raw-ip mode (without ethernet headers in the link layer) will by default be requested to use static IP addressing; not because the DHCP server in the modem doesn't work, but because not all DHCP clients in Linux can work with network interfaces without ethernet headers (e.g. dhclient can't).

Meaning, ModemManager is deliberately negotiating a static bearer for raw-ip modems because it knows linux dhcp clients can't function on an interface with no ethernet framing.

Given this, the current VyOS behavior, starting a real DHCP(v4)/DHCPv6 client against a raw-ip interface regardless of what the bearer reports - uses exactly the mechanism ModemManager's own docs say doesn't work on this hardware class, instead of the one it says to use.

What should happen is evaluation based on the bearer type (static vs dhcp) on whether to take the address,gateway info from the bearer and apply it directly to the interface (static) or to start a dhcp client (dhcp). This won't help IPv6 because there is no current DHCPv6 client with raw-ip support.