Currently the interface identifier for constructing IPv6 link-local and SLAAC addresses for PPP interfaces is the stable-privacy one.
According the RFC 5072, the interface identifier can also be set from the IPv6 Control Protocol. We don't have a specific addr-gen-mode option for that, but since EUI64 can't be used for PPP, let's override "addr-gen-mode=eui64" to mean that the IID from PPP should be used.