bug: when I enabled wan-load-balance, I found the VYATTA_PRE_SNAT_HOOK and VYATTA_PRE_DNAT_HOOK were not added to chain PREROUTING and POSTROUTING which made the load-balance SNAT rule take no affect. And finally I disabled the wan-load-balance snat rule and add my own source nat rule to fix that.
route problem: when I enabled wan-load-balance, the access to local LAN network would be routed to WAN interface and be SNATed. otherwise i exclude the destination address to local LAN network from the load-balance rule. And the problem is caused by the policy route table which only contains one default route and lacks of link route. Finally I manually added the missing part of the link route and the problem was fixed.