Obviously, the MS is just stopping the communication for no apparent reason, and the BSC is waiting until timer expiry (45s).
It sounds a lot like the “crazy” MS situation. Are you able to check whether it is a SD drop during a LU procedure ? (some vendor provides such counter)
Since the fault comes from the MS, i’m not sure which action would fix the issue… A trace would allow to know which IMSI(s)/IMEI(s) are behaving poorly.