Just to make a note, only one gateway being listed is possible if all the other gateways are on a backhaul that delivers the uplink too late to be included in the processing.
The window for the marking of the same uplinks has to be pretty narrow - at time of writing TTN is receiving 1,183 messages per second from gateways and then going on to deliver 258 messages per second.
So if it hangs around too long, it ends up with far more matching to do before it even gets to decryption, forwarding to the application server with all the steps in between.
