I've been seeing for awhile, definitely in SM but possibly not only in SM. I found, not conclusively, that if I made sure to tap each mob as I rounded it up, it'd stick to me as expected. Though they don't necessarily leash if I *don't tap them, so there must be something else up.

Maybe the leash behavior hasn't changed, but these mobs are going into a brief Evade for some reason and then returning to their spawns? Who knows!