Pretty sure I figured this out, since I was able to reproduce it as well. It has to do with caching and something Whatsapp is doing to restore the page from local data most likely, as once you clear the local data for web.whatsapp.com, it correctly requests redirecting to the correct container.

If I had to guess, I suspect that Firefox triggers the container switch dialog on the first network request, and that page is so optimized that after the first visit it loads entirely from cache and/or localstorage data without any network activity at all.

If true, it might be that while it didn't switch, it actually wasn't leaking data between containers at all, since there was no network activity. I'm not sure if a background request would have triggered a container switch dialog, been blocked quietly, or have been allowed through some root page permissions cascade.

I may be far off, and this is trivially checkable, but I'm out of time.