I didn't try to fix the rule (the generated /tmp/filter_rules_ipv6), but just added the rules of ICMP_V6 and ICMP_V6_LOCAL in the wan-event script (at "connected" event, where wan-start script also serves the purpose).
If you try to fix the rules generated by asuswrt-merlin (so to remove the...
It's because dual wan load balance mode uses another function to generate filter rules (filter_setting2), and ICMP_V6, ICMP_V6_LOCAL chains is missing in the generated rule file:
if (ipv6_enabled()) {
fprintf(fp_ipv6, "*filter\n"
":INPUT ACCEPT [0:0]\n"
":FORWARD...