Could it be that you have the gateway discovery feature enabled? You’ll find this under the network-server configuration.
This feature is sending a regular “ping” between the gateways, using the Proprietary message-type. The content is just a random token to correlate the “ping” that was sent with the gateways receiving the “ping”.
Yes, I have this feature enabled. Is this a proprietary implementation of the Chirpstack server, or is it something described in the Standard? Also, the ping originates from the AS or the NS?