Mullvad seems to be working, but some people have reported
that it is possible, it silently drops a connection but appears to
be connected.
I don't know how to check for that, though. Time to learn.
ipleak(dot)net is a much better test to see if your VPN is working correctly.
Swap proxies while the test is running to see if there's any leaks. that's the one thing that almost every VPN fails at protecting. Mullvad and proton are the only 2 I've ever used that don't leak.