Do you ever see the same packet on both gateways? Do you see all packets in the application’s Data page?
A DevAddr is not unique. TTN will use the applications’ secret NwkSKey session keys to verify the MIC, and if it’s verified it knows which application can decrypt the application payload. So, even though the payload sizes happen to be the same (and the uplink counters are always zero?): are you sure these packets are sent by your node?
If you’re not sure: maybe a different node is actually sending on 868.x, and maybe your full gateway has erroneously been configured to use the 868 settings? (868 and 915 MHz use different antenna connectors, but maybe the hardware is capable to handle both, so using the wrong settings might be possible?)