That's not even the way to do failover, you would use Gateway Groups for that. ;) In my understanding, your disabled rule would never be executed (even if it's enabled) as it matches exactly the same pattern as the rule that comes before.
When you say "physically disconnect the Fibe cable", do you really mean the fibre cable or actually the cable between the pfSense box and your modem? I could imagine that your global routing configuration is somehow in a way that WAN_DSL_PPPOE kicks in as the default gateway. I maybe wouldn't expect that, but I could imagine, that when you physically remove the network cable (which is not the typical real-world outage scenario) from the pfSense box (so that the network interface changes its whole state to disconnected), that the gateway WAN_FIBRE_PPPOE gets removed from the system in a way that it has the same effect as if it wouldn't exist at all and the then active default gateway (WAN_DSL_PPPOE) is used. Just a theory. ;)