there are two options you may do:
1) extend the transient period time - which is probably what yo don't want to/
2) use "Reserverd" status for IPs with manually assigned IP addresses.
From the scenario above, you are saying that even though transient period expires then some device may be offline and you suppose that this device has manually assigned IP address - static assignment. According to best practices, you should use "Reserved" status for that IP address because of two reasons:
- you need to track manual/static assignments
- you need to let other admins/IP users know that some machine has an IP address reservation so they can't simply assign the same IP to another MAC address.
Transient status is there mainly for non-reservation cases, especially for DHCP environments.