It is a well known problem of the ZS, since older versions. My workaround was to split the address space 0.0.0.0/0 in 2 (0.0.0.0/1 and 128.0.0.0/1) and send http and https for the first address space via WAN1 and the second via WAN2.
Routing and Bridging Firewall Solutions
It is a well known problem of the ZS, since older versions. My workaround was to split the address space 0.0.0.0/0 in 2 (0.0.0.0/1 and 128.0.0.0/1) and send http and https for the first address space via WAN1 and the second via WAN2.