In Neutron, "Protocol=icmp" is allowed to be configured with "Ethertype=IPv6" for backward compatibility reasons.
When such a rule is configured, Neutron IPTables Firewall Driver (i.e., reference implementation) treats this as a request for ICMPv6 [1] and configures a rule accordingly.

Currently, to validate some IPv6 scenario tests, tempest uses ping6 to check the IPv6 connectivity between VMs spawned on a network.
Before validating ping6, tempest configures a Security Group rule with ethertype='IPv6' and protocol='icmp'.
Since ACLService is not doing something similar to IPTables Firewall Driver, the IPv6 tempest scenario tests are failing.

[1] Snippet from iptables_firewall.py
def _split_sgr_by_ethertype(self, security_group_rules):
ipv4_sg_rules = []
ipv6_sg_rules = []
for rule in security_group_rules:
if rule.get('ethertype') == constants.IPv4:
ipv4_sg_rules.append(rule)
elif rule.get('ethertype') == constants.IPv6:
if rule.get('protocol') == 'icmp':
rule['protocol'] = 'ipv6-icmp'
ipv6_sg_rules.append(rule)
return ipv4_sg_rules, ipv6_sg_rules

Ref: https://github.com/openstack/neutron/blob/master/neutron/agent/linux/iptables_firewall.py#L379