I think he's saying that the active unit comes back online, not back as active. In this case, if the traffic had already been failed over to the other unit and working fine, it shouldn't be an arp problem as the formerly active unit will (should) not be responding to the virtual arp requests. It could be a stp problem, but only if you have the ports the LTM connects to configured as network ports instead of host ports. These should always be host ports unless you're using vlan groups. And if not, no reason to have stp enabled.