Does VPN have auto locking of network connections if VPN fails?

I would treat that as a fail-open case until you can prove otherwise. In the Mozilla VPN app, check whether there is an explicit kill switch / block-network-on-disconnect setting for your platform; if it is not there or was off, don’t assume the VPN will stop all traffic after a crash. On Ubuntu I’d also test with a simple controlled case: connect the VPN, confirm the public IP, then kill/restart the Mozilla VPN process and watch whether traffic still leaves via the normal interface. If you need a hard guarantee, use an OS-level firewall/NetworkManager rule that only permits traffic through the VPN interface, because that survives better than relying on the app UI.

Thanks. This I already knew. Other VPN use a deployed side program to lock the networking if the VPN fails. I will wait to get some information directly from Mozilla techs.

Mozilla VPN as a paid product gets dedicated support via private tickets — please submit your question with your Mozilla VPN app by going to Settings > Help > Contact Support. Alternatively, you can sign in to the Mozilla account connected to your subscription here (if not already) and then use this link.