On slave configuration when using AXFR to send presigned zones to slaves, the auth server does not serve DNSSEC records from the zone.
Workaround is to setup SQL backend with dnssec schema, as explained it http://doc.powerdns.com/dnssec-modes.html#dnssec-bind-hybrid
See comment above. Also, ticket #369 is relevant to this question. Closing this ticket as 'worksforme'.