Class-C downlinks are not transmitted without time synchronization

Class-C downlinks are not transmitted without time synchronization.

I am using a Kerlink iFemtoCell.
ChirpStack is running on the gateway itself.
The ChirpStack version is v4.13.0.

When the gateway is synchronized with NTP (chrony), Class-C downlinks are transmitted correctly.
However, if time synchronization is not performed, Class-C downlinks are not transmitted correctly.
Instead, they remain in the queue as if the device were operating in Class-A.

I am planning to use this system in a closed network without access to an external network.
In this case, is there any way to make Class-C downlinks work correctly without using NTP (chrony) time synchronization?

The following situations allow Class-C downlinks to be transmitted correctly:

When the gateway is time-synchronized using NTP (chrony), Class-C downlinks are transmitted correctly.

If I delete the device registration and register the device again while time synchronization is not active, Class-C downlinks are transmitted correctly. However, after rebooting the gateway, Class-C downlinks are no longer transmitted.

I also tried setting the time manually using the Linux date command, but Class-C downlinks were still not transmitted.