Provide the output of /ip route print, twice, once when in the “normal” state and once when you have the ISP WAN cable detached, as it will be more clear what actually happens.
As a side note, and JFYI, another possible approach (IMHO simpler):
http://forum.mikrotik.com/t/simpler-failover-for-two-gateways-i-found-working/169108/1