Well as one would put it “it works as designed”. 
However you can easily “exclude” some traffic from the WAN load-balancing:
set load-balancing wan rule 1 exclude
set load-balancing wan rule 1 protocol all
set load-balancing wan rule 1 inbound-interface <lan-interface>
set load-balancing wan rule 1 destination address <ip-address-range>
However as some other user observed sometimes determining the actual IP address range for a bank is a daunting task…
Therefore a better solution is to designate a particular IP in your LAN as “banking only”, and set an exception on that by using source instead of destination in the snippet above. (Then you can manually configure that IP while you are using your bank.)
Please note that if you have a more complex setup, that includes policy-based routing, then the WAN-load-balancing feature doesn’t work properly. You can consult the following post of mine that gives a “hacky” solution.