PC client, Windows 10, Steam.

Apothecary Stompers were able to attack on the round they entered.

They were all summoned by Loot, Exuberant Explorer's mana activated ability.

The Apothecary Stompers did display the summoning sickness dark swirl animation when they came into play, then the new summon stacked on the previous Stomper, the swirl animation disappeared, and they were able to attack.

Didn't observe any buff in effect style Haste text on the Apothecary Stomper cards.

The board was pretty sparse when this happened so I don't think this was just an overlooked effect on my part. I was on the receiving end, and rather surprised the first time, but by the third summon I was looking pretty hard for what was going on, and couldn't see any card effect on the board that would explain the behavior.