To work around this at present I am setting the following additional configuration to prevent losing IPv6 connectivity frequently:

Sorry, another followup, rather than duplicating the control logic in both interface.py and in vyos-netplug-dhcp-client (and leaving yourself open to another logic divergence in future), you could consider just checking for presence of the config file /var/run/dhcp6c.eth0.conf in vyos-netplug-dhcp-client when determining if the service should restart. That file only exists if dhcp6c is required on that specific interface, and that means the logic for dhcp6c presence is determined only by interface.py and vyos-netplug-dhcp-client just follows its lead.

Relevant section of interface.py: https://github.com/vyos/vyos-1x/blob/5845c4b1c50bc0335284cfb3306e0a91de3efd40/python/vyos/ifconfig/interface.py#L1822-L1824

Just ran into this with 1.2 RC9. eth0 and eth2 swapped devices, presumably due to differences in the timing of device init in the new kernel version, meaning DHCPd ran on the WAN interface, which the upstream won’t appreciate. Having proper static device naming would have prevented this.

@gadams I could have sworn I'd put $ifname in there, but the git history doesn't lie; apologies for missing that and thanks for catching.

Resolved by waiting for IPv6 DAD to complete in scripts/vyatta-dhcpv6-client.pl. The 'configure failed!' message no longer appears, dhclient starts and the configuration nodes are not deleted.
Typically it only has to wait 1 second for DAD to complete, so on some systems/reboots you would never have seen this issue.

The relevant Debian bug is https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=834928
From this report, the issue is in ifupdown and is supposedly fixed in 0.8.11. Unfortunately, this version is not available in Jessie without resorting to backports.

Some further investigation: