As Matthew suggested, simply running this through debugger should explain how AT... events work. You'll also see in debugger that inside AT... ENDAT only certain fields have values and others are replaced with the asterisks. These events are tricky.
I didn't get how sy-tabix is involved here exactly or even what are you trying to achieve, sorry. It does seem like you rather need COLLECT but it's not really clear what's the goal here. You might want to clarify.