TTL expire message

Hello,
We are testing our broker config link-heartbeat-interval. We create the scenario on network failure on 1 link, on a 2 links network.
Message's TTL is 18 seconds. We capture message at the sender and receiver's exchanges. We measure the message delivery rate by matching up message ids at both ends.
For link-heartbeat-interval = 2, there is no message lost. link-heartbeat-interval = 10, there are message lost. However, for the both configs, we still see discards-ttl-expired increases with qpid-stat.

From my understanding of the acquire/acknowledge process of a message, when the message is marked for redelivered, the other consumer (2nd link) can acquire and acknowledge the message. But from what i'm seeing, the 2nd consumer got the message but still not able to acknowledge it.

Could you give me a better understanding of this whole process? And how can I avoid having the discards-ttl-expired messages.
Thank you.
Huong,