after further testing, I realized balancing is working if either client IP and/or destination IP change.
It means that, if from the same client behind the LAN I access to the same server IP, I always traverse the same WAN. If from that very client I access to another server IP, the WAN alternates.
Different clients to the same server IP also alternates.
This shall fix the HTTPS/cookies problem, but I’d like to know if this algorithm can be customized and where the logic lays.
In fact, I’d like to specify rules based on bandwidth in use on the channel instead that only relative wheights (I’d like to use WAN2 only if capacity on WAN1 exceeds 80% of its limit that is specified in the QoS “Global Bandwidth” settings), and then also alternate WAN on the same client-server if necessary.