It sounds like an issue where IPtables (v6) tries to start while another process of IPtables (probably v4) is already managing the rules.
I've done a bit of research and it seems like TripleO already use --wait option when using IPtables, since the iptables 1.4.20 (we have 1.4.21 in RHEL 7.4):
https://github.com/puppetlabs/puppetlabs-firewall/blob/b0e45f8ca06874b500b65dd36ab1dd838aed5826/lib/puppet/provider/firewall/iptables.rb#L651-L657
Since you hit that at reboot, I think we need to change the iptables.init (in packaging) to start the process with --wait if possible, so we avoid a race condition where iptables starts in the same time as ip6tables.
At this stage, we should probably reach out IPtables maintainers to get their feedback.