And yet, a large portion of big websites using SPAs don't seem to have functioning back buttons. If a billion dollar company can't get it to work properly I doubt anyone else can.
Back buttons in web apps are hard regardless of SPA or MPA.
I've seen many MPAs fail, for example after checkouts or form submissions.
The problem is just more prominent with SPAs because they usually feature more complicated navigation.
Perhaps for privacy reasons, you don't get much control over the built in history: you can push, replace and pop.
Maybe you could implement your own history, but then you need to somehow keep that in sync with the browser.
If someone knows a good approach, I'd be interested.
Billion dollars companies rarely delivers high quality software, especially in the UI area.