Mullvad's security team should have found this problem on their own, and as soon as it appeared:
Inspect security empirically - you might think that your security must work, but that means nothing; you must investigate empirically: All data going to the Internet must pass through the gateway. Collect the packets on the gateway, not on the device, and inspect them for leaks. Finding leaks should be trivial at that point.
The only trick might be cellular connections: We don't know that leaks aren't unique to cellular connections. I know cellular gateways can be setup, but are the packets inspectable at a level that will reveal leaks?
Inspect security empirically - you might think that your security must work, but that means nothing; you must investigate empirically: All data going to the Internet must pass through the gateway. Collect the packets on the gateway, not on the device, and inspect them for leaks. Finding leaks should be trivial at that point.
The only trick might be cellular connections: We don't know that leaks aren't unique to cellular connections. I know cellular gateways can be setup, but are the packets inspectable at a level that will reveal leaks?