Comment 3 for bug 1914037

IMO the problem is in the devstack's configuration (or tempest test) as it is using same IPv6 subnet 2001:db8::/64 for public network (devstack creates it) and later in the tests for private networks.
Patch https://review.opendev.org/q/Id5d8ac09a38c656619f88a6f87b8f384fe4c55a8 just fixed bug in Neutron and improved checks of the subnets to prevent such situation. I'm adding tempest as affected project as IMO bug is there, not in the Neutron code.