Look at the RFM69 libarry packet structure:
Important to understand that Nodes on the same frequency will share the same spectrum regardless of the settings.
In the packet structure above the network ID would always match since its outside the encrypted part.
You would then decrypt the message (which would be gibberish because the key does not match), but the destination node ID would be random (whatever decryption will decrypt that byte as).
So whenever that byte happens to match the listening node it would accept the (gibberish) message. Since it's garbage it won't really matter. But if you want to keep channels truly separated use different frequencies. Or at least different network IDs so the message would never decrypt if the network ID byte does not match.