Like most if not all ISPs, Zen drop PPP sessions sometimes for load balancing or maintenance purposes. However, a 30-60 second outage sounds more like one end determining from a failure of LCP echo request/reply (like a ping, but at a lower layer of the protocol stack) that the link has failed before tearing it down and re-establishing it.
Unfortunately, I can't get any meaningful PPP session data out of my recent logs, because there are numerous outages over the past fortnight relating to me changing one of the two main network switches here, as well as some debugging and development work on the PPPoE functionality of the pfSense firewall I use.
I've added RFC 4638 support to pfSense, so I'm now running with a MTU of 1500. I have also made pfSense re-establish DHCP-PD on a linkdown -> linkup transition, as Zen's trial IPv6 service requires you to delegate the prefix before a route is installed for your IPv6 /48 block.