I had a similar problem, when using a failover bonding, sending packets to a bridge at the other side through two different tunnels, each one using a different provider wan link.
the bridge at the other side was sending back arp traffic sent by the bonding dead link arp detection, causing unuseful traffic on the spare link.
A solution is to use IP target failover detection instead of ARP, this is implemented in the bonding driver, but the switching response time is in the second range instead of millisecond.
Using the bonding driver in arp detection mode is a must for VoIP tunnels, it does allow to switch a dead wan link in about 40 ms, almost not noticeable on a phone call.
There is a setting in the bonding driver that give the possibility to setup the arp time between each request.
It would be nice to have access to those bonding driver settings in the GUI, as well as giving access to other bonding modes (there are about 5 modes if i remember correctly).