Monsters no longer despawning?

In the past monsters would despawn once you moved more than about 2 screens away.

However I've noticed monsters no longer de-spawning regardless of how far away I move. I've tested this multiple times today with TZs. I teleported past all monsters in TZ catacombs and went straight to kill Andariel and waited in Catacombs Level 4. Once the TZ rotated to the next I went back to Level 3 and 2 and the monsters I had originally spawned along my route were still Terrorized.
I did the same test with the Tower with the same results.

Can anyone else confirm this? Was this already discovered and talked about and I just missed it?

I wonder if this has anything to do with how the Heralds work. The 'receiving' and 'placing' of a Herald token. Making TZ monsters not de-spawn would stop people from gaming Herald spawning.

Edit: The more I think about it, this updated no-TZ-despawn mechanic wouldnt affect the receiving of a ' Herald token', since you need to kill to get one. But it would affect the 'placing/using' of a Herald token since that requires 'discovery' of the monster.