active-backup bond having both interfaces active

we use a bond with BONDING_OPTS mode=acitve-backup
however the investigation of varying amount of packet loss showed that somehow both interfaces are active and visible with the same MAC on the LAN switch. The passive slave is receiving the packets and is not responding appropriately (ICMP echo reply), so they are getting lost.
The setup is largely the same on 5 physical servers, but the issue is present only on one of them.
Please suggest an approach to solve.