Well, it happens with aMule's unit tests ("make check"), too. Basically, it's the "unsigned long long" format specifier "%ull" that doesn't work properly in wx's wxVsnprintf. I'm not certain if the "unsigned" is required; it might be that the "long long" format specifier "%ll" is also broken.
EDIT 1: I looked at the wxVsnprintf code a bit. It's complicated, so I'm not sure I followed it correctly, but I don't think it will work with long long format specifiers on any platform. The only reason long longs ever print properly is that the vast majority of platforms use the native library, not wx's implementation (as we've discovered on the Mac).
EDIT 2: Hmm. I just tried and aMule's unit tests don't compile these days. Anyway, the problem is as I've described it above.