Browser tabs are the fault here and browsers are trying to be OS environment, so Alt+Tab is useful for major task switching. I agree it's inconsistent and annoying, but I like Alt+Tab as a way to try to find the window I'm writing that email to someone.

Android and Chrome worked like this for a hot minute too. I assumed the idea was to promote webapps to look like they're first-class citizens, but in practice it's just bizarre and confusing UX.

I hate this too. You can turn it off. In Settings, go to System->Multitasking and change "Show tabs from apps when snapping or pressing Alt+Tab" to "Don't show tabs."