Thanks for your reply, that made it work.
However, I still don't understand what's going on. It hardly seems reasonable to have to flush the output buffer every time, especially if you have a lot of back and forth I/O on stdout.

Has this functionality changed between Qt versions - why haven't I noticed this before?
What exactly determines the order in which the I/O is printed - is it even deterministic?
Why does it "work" on some computers but not others?

I have much more experience with iostream than QTextStream (which is why I like cin and cout), and I've never encountered this. Does iostream handle buffering in a fundamentally different way, or does this same thing sometimes happen with iostream?