OpenWrt "Chaos Calmer" 15.05 released

Perhaps the NAT rules for the WAN port are erroneously being applied to packets traversing the bridge? IIRC the decision of whether to mangle a packet for NAT is separate from the decision to route the packet out over any particular interface, and if you have multiple interfaces you need to include a condition (e.g. in my case it's "-A POSTROUTING -t nat -s $LOCAL_IP_RANGE -o $WAN_DEV -j MASQUERADE") to prevent NATing of internal packets. If the "-o $WAN_DEV" condition were omitted it could cause the symptoms you're seeing.