You need rules for the forward chain. Here is my current v6 firewall:
/ipv6 firewall filter
add action=accept chain=input comment="RB-FW: Allow ICMPv6" disabled=no \
protocol=icmpv6
add action=accept chain=input comment="RB-FW: Accept established connections" \
connection-state=established disabled=no
add action=accept chain=input comment="RB-FW: Accept related connections" \
connection-state=related disabled=no
add action=drop chain=input comment="RB-FW: Drop invalid connections" \
connection-state=invalid disabled=no
add action=accept chain=input comment="RB-FW: UDP" disabled=no protocol=udp
add action=accept chain=input comment="RB-FW: From wlan" disabled=no \
in-interface=wlan
add action=accept chain=input comment="RB-FW: From LAN" disabled=no \
in-interface=lan
add action=accept chain=input comment="RB-FW: From Hotspot" disabled=no \
in-interface=wlan-Public
add action=log chain=input comment="RB-FW: Log everything else" disabled=no \
log-prefix="IPV6 INPUT DROP"
add action=drop chain=input comment="RB-FW: Drop everything else" disabled=no
add action=drop chain=forward comment="MAINFW drop invalid connections" \
connection-state=invalid disabled=no
add action=accept chain=forward comment="MAINFW Accept UDP" disabled=no \
protocol=udp
add action=accept chain=forward comment="MAINFW accept ICMPv6" disabled=no \
protocol=icmpv6
add action=accept chain=forward comment=\
"MAINFW accept established connections" connection-state=established \
disabled=no
add action=accept chain=forward comment="MAINFW accept related connections" \
connection-state=related disabled=no
add action=accept chain=forward comment="MAINFW forward from wlan" \
disabled=no in-interface=wlan src-address=2001:470:xxxx:2::/64
add action=accept chain=forward comment="MAINFW forward from lan" disabled=no \
in-interface=lan src-address=2001:470:xxxx:1::/64
add action=accept chain=forward comment="MAINFW: forward from wlan-IPv6" \
disabled=no in-interface=wlan-IPv6 src-address=2001:470:xxxx:3::/64
add action=accept chain=forward comment=\
"MAINFW forward from Hotspot, only to wan" disabled=yes in-interface=\
wlan-Public out-interface=sit1
add action=drop chain=forward comment="MAINFW drop bit torrent on hotspot" \
disabled=yes in-interface=wlan-Public protocol=tcp
add action=drop chain=forward comment="MAINFW drop anything else" disabled=no