Your node configuration is not completely clear to me: if you configure personalised addresses, how can it do a join on another network? Also, how can it fail to do ABP?
I think the confusion is related to the keys. TTN uses a default key for the demo backend right now, and maybe the other network uses the same key. A node would then 'work' on both networks.
In the new architecture, each node will have a unique NwkSkey and AppSkey, and this should not happen.
Out of curiosity, which 'proprietary' network are you referring to?