Update. I have two ethernet ports on the RP: the original one eth0, and another one on a USB-to-ethernet adapter as eth1. I have been using eth0 as my WAN connection, eth1 as my LAN. With that setup, I have been getting problems getting PPoE PADI packets out to the WAN. So, I just tried swapping them around: the USB ethernet adapter is now my WAN port, the builtin ethernet is now my LAN port. I've rebooted the RPi at least 6 times now, and each time pppoe-wan has come up immediately, and I can see the IA prefixes coming up okay.
Assuming this behaviour stays consistent, it more and more looks like a issue with the device drivers, perhaps. Yes, I know that on the RPi, ethernet hangs off the USB bus. So two USB-ethernet adapters, one apparently ok, one not. Hmmmm!