the "set link-failed-signal enable" moves the port down for one second.
The thing is that connected to FGs I hace two MCLAG switches connected with an aggregate of 2 ports each switch to FGs. I mean, port1 of switches to FG active and port2 of switches to FG standby, all four ports in the same MCLAG aggregate. In the FG side, one aggregate with ports A and B monitored for failover.
FG Active portA --> Sw1 port1 (MCLAG)
FG Active portB --> Sw2 port1 (MCLAG)
FG Standby portA --> Sw1 port2 (MCLAG)
FG Standby portB --> Sw2 port2 (MCLAG)
If Sw1 port1 fails, the FGs move the active unit making the standby as primary, but SW2 port1 continues sending traffic, and loosing it.
Finally I could solve the situation changing configuration in FG aggregate interface:
config system interface
set lacp-ha-slave disable --> With this command the stanby unit has the ports level 1 link up but switches ports connected to standby unit are in suspended mode even in failover I explained before.