The timestamp is an internal timestamp to the gateway. E.g. it starts counting at 0 on boot and then keeps counting. The issue probably is that the downlink timestamp already occurred when it received the downlink transmission request. Probably this means that the roundtrip between the uplink and downlink is > 1s. When your gateway is operating on an unreliable network, you could increase the rx-delay in the loraserver.toml configuration for example.