Which router is the gateway registered at? And are the applications using the same region for their handler?
Indeed, you should not do that unless you’re sure your forwarder does not require a specific gateway ID during registration; see Passing on a gateway to a new owner. Also, I doubt it would help: even if you would mess up a gateway’s ownership then TTN would probably still accept and route its data. You would not be able to see that in TTN Console if TTN cannot verify the gateway is yours, but the device’s application should still get it forwarded.
Just to be sure: why do you explicitly mention join requests? For a gateway these are just uplinks.
Also, just to be sure: it’s not only about not seeing anything in the gateway’s Traffic page in TTN Console, right? That is known to be buggy. So: you’re not getting any device data (received by that gateway) through, e.g., MQTT or another API?
Do you have devices for which uplinks were received by multiple gateways? If yes, are these indeed not showing this gateway in the metadata anymore either?
Good thinking! 
If, however, you see some drop, then maybe the status messages account for more 4G data than the actual uplinks. Can you also access the Kerlink’s command line to see its logs?
I think I saw this problem reported earlier, for which restarting the packet forwarder actually helped. But then, of course, you would be in the dark as for the reason…