I might as well add my thoughts to the mix.

First of all. I'm pretty certain that Asmo died in the palace in Camelyn. It isn't likely that he would pass through a gateway without noticing and even more unlikely that he would walk into T'A'R.

So the question we should be asking is, what's special about the Camelyn palace?

One possibility becomes clear if you think about what is known to interfere with the DO's ability to catch souls, namely balefire.

So I belive that Rand's and Rahvin's use of balefire distubed the pattern, making it a little bit harder for the DO to catch Asmo's soul in time.