Driver monitor interface's link state (physical signal) and notify the networking subsystem when that link state changes.
In case link present between your server and main switch, system not failover to another interface, even all other link on switch will broken.
After performing changes in /etc/default/mpathd you need restart in.mpath daemon ( reload configuration) pkill -HUP in.mpathd
I think this is already covered in the documentation:
As far as I know if the interfaces are no part of an IPMP group the daemon will not detect failures.