How odd. I'm experiencing this same issue precisely as described above and I am also using the else clause.
Also, I tried the double logging method as mentioned above like so, just before my if statement to log pass or fail:
UIALogger.logMessage("My Story Title: " + titleDisplayed);
UIALogger.logMessage("Innocuous Message.");
And got a FAIL, which is fine--I wanted anything other than the "Issue" result and the "script ended without expliciting" message. So, I then commented // the second logMessage statement and re-ran the test and this time got the "Issue" and "script ended without explicating" again. 
So, his above tip has been verified as real. 
I'm running Xcode and Instruments version 4.6 with the iPhone Emulator running as a 6.1 project.
---Update--
I cannot get pass/fail to work properly no matter what I do. On one run I get the enraging "Issue: Script ended without explicating " message in my log, and then if I make any changes to the script--even if it's just to add an extra whitespace, and save it and then run again--I'll get a good pass or fail result. 
But if I then immediately run the test again without any changes at all I'll get the "Issue: Script ended without explicating" message and this will not go away until I make a meaningless change in the script and save and then run again and then MAYBE at this point I might get a decent pass/fail result (doesn't always happen), but if I run the script again I will get "Issue: Script ended without explicating" again.
What the heck? I've actually taken the time to reinstall Xcode 4.6 but no luck in getting the behavior to change. I'm profoundly frustrated. This appears to be a bug in Instruments. Is there a way to report this to Apple?