Does Mullvad have a kill switch in the Android client? I think, following your own reasoning, you’d have to do quite a few gymnastics to argue that it does (by your definition). On GrapheneOS it can be resolved, but on all Android devices (which are the vast majority), every time you connect to a Wi-Fi network you suffer a data leak.
Can Mullvad fix it? No. And Proton with respect to macOS? Yes. There’s no argument here, I’m conceding your point, but please think a bit more globally: this isn’t about defending one company over another, I couldn’t care less.
What’s the point of using a VPN if it’s going to have leaks? If you want to be anonymous, you use, for example, TAILS (not Proton, not Mullvad, not iVPN). If what you’re concerned about is hiding the pages you visit from your ISP, in my opinion ProtonVPN will serve you perfectly well on macOS despite those leaks when switching servers.
As was said above, but it seems the eagerness to cross Proton off the list is overriding your ability to actually listen to the arguments:
And I keep bringing this topic up precisely because, as @Jonah said when closing the Remove iVPN thread, this touches on a much broader discussion: