FortiOS 7.4.8 – IPv6 captive portal MAB: no RADIUS request sent

Hello,

has anyone experienced issues with RADIUS authentication for IPv6 clients on a captive portal interface?

Setup: FortiGate 200F, FortiOS 7.4.8, interface with security-mode captive-portal + security-mac-auth-bypass enable.

IPv4 MAB works flawlessly — FortiGate sends an Access-Request with the client MAC as username, RADIUS responds with Access-Accept, and the auth entry appears in diagnose firewall auth list. No issues.

config authentication rule
    edit "TEST Radius"
        set srcintf "50 Lan"
        set srcaddr "all"
        set srcaddr6 "all"
    next
end

For IPv6, FortiGate sends no RADIUS request at all (confirmed via tcpdump). The diagnose firewall auth ipv6 list remains empty regardless of configuration.

What was tested without success:

1. config authentication rule with srcaddr6 "all" — CLI accepts and saves it, but the daemon never processes IPv6 sources.
2. Framed-IPv6-Address (RFC 3162) in Access-Accept reply — FortiGate receives the attribute (confirmed via tcpdump, packet length increases accordingly) but ignores it, IPv6 auth list stays at 0.
3. Fortinet-Client-IPv6-Address (VSA 4, vendor 12356) in Access-Accept — same result, attribute received and silently ignored.
 

Is this a known limitation of 7.4.x? Has anyone confirmed this being fixed in 7.4.9+ or 7.6.x?