I had a Pelt Collector in play with its base value of 1/1. I then played another creature whose power was greater, adding a +1/+1 counter onto Pelt Collector. My opponent then played a Grasp of Darkness on the creature that just entered, destroying it. The +1/+1 counter was removed from Pelt Collector, which shouldn't happen.

My following turn, I played another creature whose power was greater and it added TWO +1/+1 counters onto Pelt Collector.

Seems like some kind of check for the newly-entered creature's power was happening again after it had already resolved.