I've found a rather strange behaviour in my phpBB 3.0.11 installation. Apparently, whether a global thread is shown to be closed or open depends on the forum from where one is opening the thread. I opened a global thread in the General forum, which is open for everyone, and if one opens the thread from there, it is shown to be open. However, if one opens it from the News forum, which is closed, the thread seems to be closed. Opening the thread from the "Search new posts" (or whatever it's called in English; the route is search.php?search_id=newposts) also shows the thread as closed. This is annoying, since I'd expect the thread to be open for everyone, regardless of which place they come from.

So, basically:

viewtopic.php?t=1783 shows the thread as closed (even though it was opened in an open forum).
viewtopic.php?f=3&t=1783 also shows the thread as closed, with f=3 being a closed forum.
viewtopic.php?f=56&t=1783 shows the thread as open, with f=56 being an open forum.

If it's any help, the forum with ID 1 is a category, and the forum with ID 2 is closed. The open forum with the lowest ID is 5.