Apologies in advance if I'm missing something. :)
Marked 3.0.3 1127.
I have Settings > General > Window > Open documents in tabs = unchecked
When I open a second document it opens in tabs. Doesn't seem to matter how I open, Finder double-click, right-click Open..., Marked File | Open.
I've toggled the settings checkbox a few times.
It's completely possible, I'm doing something wrong or I'm not understanding the intended behavior.