Just discovered today that the one cnPilot we have behind a bridged SM is also having this problem. So all ePMP and our one cnPilot's PPPoE client craps out while the 450i , Canopy and a horde of $30 belkin and netgear routers work just fine.
Edit: Oh and I forgot about the packetloss ... below the ePMP radio and network do not change (other than putting the ePMP in bridge mode for the other two tests). Also of note is that the Pings when the ePMP was in PPPoE mode were being sent every 2.5 seconds. When I started to do the other test and realized that I set the pings to every .5 seconds. So Windows PPPoE and $30 router PPPoE lost zero packets in hours sending pings every .5 seconds.
PC ---> ePMP doing PPPoE ---> Network ---> PPPoE Server ---> Cloud = 30%+ PL
PC windows PPPoE --> Bridged ePMP ----> Same as above = not a single packet dropped in hours
PC ---> $30 router doing PPPoE --> Bridged ePMP ---> Same as above = No packets dropped in hours