BigIp HA and APM session's expiration timestamp reset

We are facing a strange behavior regarding APM (BigIP VE edition) and session when HA (failover) is configured in active / passive mode.

When a failover happens (primary BigIP no more available), all sessions are correctly retaken on the new active BigIP (secondary BigIP).

However,

Expiration

date is reset to

time of HA switch + APM TTL

on the newly active BigIP.

Example:

Session1:

BigIP1 (Active): expiration T1`
BigIP2 (Standby): expiration T1`

Session1 after failover at time T:

BigIP1 (Standby): expiration T1
BigIP2 (Active): expiration T2 (T2 = T + APM_TTL)
 

The issue is that reseting the expiration date of all APM session is substantially increasing their number because of idle sessions, we need an APM session TTL of 1hour...

Why is BigIP resetting the session Expiration timestamp?

Have we missed a configuration option in APM?

Thanks,

JSC