TTN cannot really tell where your gateway or device is. Using a non-EU868 gateway on ttn-router-eu may cause weird problems with wrong channels for downlinks, but in your case it’s the uplink that’s using the wrong frequency. For that, the device is to blame: devices just transmit, they don’t know what gateways are around.
You’ll have to check the firmware again.
And make sure to keep at least a few meters between device and gateways: even if the device is using US915, maybe a EU868 gateway erroneously picks up something due to overloading of its receiver? That surely is the case for channels within a single frequency plan, when devices are too close; I don’t know if the same problem could show between different frequency plans. In that case your US915 gateway would be broken, as it’s not receiving anything. Make sure to look at the logs of your gateways; don’t only rely on TTN Console.