To repeat what others have said, this is not unspecified behavior, but rather undefined. This program can legally output anything or nothing, leave n at any value, or send insulting email to your boss.
As a matter of practice, compiler writers will usually just do what's easiest for them to write, which generally means that the program will fetch n once or twice, call the function, and increment sometime. This, like any other conceivable behavior, is just fine according to the standard. There is no reason to expect the same behavior between compilers, or versions, or with different compiler options. There is no reason why two different but similar-looking examples in the same program have to be compiled consistently, although that's the way I'd bet.
In short, don't do this. Test it under different circumstances if you're curious, but don't pretend that there is a single correct or even predictable result.
