Conditions
FTD being managed on-box, Firepower Device Manager FDM.
Workaround
Manage the FTD with Firepower Management Center (FMC).
Further Problem Description
FTD being managed via FDM should include DHCP Relay configuration, as well as DHCP Server lease timeouts.
FTD configured as DHCP Relay is currently supported only on FMC and might be needed when an external DHCP Server is already configured on a network.