FortiSwitch trunks best practices across multiple switches?

I have a site where we tried to stagger trunks between multiple switches, for redundancy should one fail.  We were hoping STP would deal with keeping things straight, however when we rebooted switch #2 in the below diagram, #3 went off permanently, as if it was only ever trying to look for a connection from its port50 and not failing over to port49.

On other sites we have switches that have dual trunks that FortiLink takes care of LACP automatically, like this:

Switch1-1 # show switch trunk
config switch trunk
  edit "4FPTF20001720-0"
    set mode lacp-active
    set auto-isl 1
       set members "port49" "port50"
    next
end

But in the below picture example, here is the same output for Switch #1:

Switch2-1 # show switch trunk
config switch trunk
  edit "8FFTF21004328-0"
  set mode lacp-active
  set auto-isl 1
    set members "port48"
  next
end

And here is the second switch's output in the below example:

Switch2-2 # show switch trunk
config switch trunk
  edit "8FFTF21004165-0"
  set mode lacp-active
  set auto-isl 1
    set members "port48"
  next
edit "8FPTF20001565-0"
  set mode lacp-active
  set auto-isl 1
    set members "port49"
  next
edit "G101FTK19006567"
  set auto-isl 1
  set fortilink 1
    set members "port47"
  next
end

I'm just curious if this is not best practice, as something is definitely wrong-- a reboot of switch #2 takes #3 offline until the latter is power cycled.

This setup is below: