Ugh, that seems like a bug which should be fixed. It's pretty inconvenient for users. :(

Probably not trivial since the back button might be linked to history and the container is doing it's job in isolation.

I would expect there is a setting to force opening the container in a new tab if you want to be able to go back.

I think a new tab for the container and leaving the original tab open would be the least surprising/inconvenient behaviour.

There must be since that's how it works for me. Either I changed it, or it stopped being default at some point.

You can change that in the settings. It's under 'Tabs' in the 'General' section of 'Settings' from the hamburger menu in the top-right (check/uncheck "Enable Container Tabs").

I quite like having that on by default.

The extra security aspect seems important, but the browser shouldn't just drop the user's previous history (for that tab) just because it's isolating things. :/

Hopefully this is just a case of "early stage feature" and bugs like this will be fixed. :)