Whenever Tasha, Unholy Archmage's first ability gets activated, it creates the effect that should trigger whenever and opponent attacks, or disappear when your next turn begins.
These last couple of days whenever I've been playing her in brawl matches, the effect does not disappear whenever my next turn begins, and instead creates a stack of these effects.
They all trigger when an opponent attacks, creating the moment when Tasha puts more than 1 -1/-1 counter on each attacking creature.

Thus far this has happened every time i use Tasha's ability in Brawl. I suspect in theory you could stack it infinitely to cause any attacking creature to die immediately.
Im playing on PC, Windows 10 OS.