In VyOS 1.2.6 the PPPoE server config version is still 0 (https://github.com/vyos/vyatta-cfg-system/tree/cae9145346b60ebd6251643a5ce0ceed2a0f2364/cfg-version)
But the migrator references version 1 https://github.com/vyos/vyos-1x/tree/94355927bfa21cda01e7db54060cf0292bbacd43/src/migration-scripts/pppoe-server
In addition the version was bumped once more