pppd discarding proto 0x8021 in phase 5

Recently I have been seeing a problem on multiple devices where pppd never completes. It looks like the ISP thinks that pppd is ready for IPCP, but pppd isn't and so discards the IPCP packet (discarding proto 0x8021 in phase 5) and things just go around in a loop like this forever until the connection is interrupted and it starts again. On starting again, it will either connect normally, or will get itself into the same loop and never progress. Has anyone seen that before? It seems to be a recent thing - I've only seen it in the last few months. James