Full details:

Enemy playing Garruk, and I played Jace.

Garruk summoned Krav, Unredeemed. I played Evil Twin, targeting Krav. Garruk summoned 2 Sadistic Mindflayers, pumping their Krav to 12/12 and their Krav died. I followed with two Prodigal Students and targeted one of the Sadistic Mindflayers with Agony Warp. Garruk summoned Wildshape Brontosaurus, and on my next turn, I targeted my Krav with Heat Shimmer and the enemy blocked, killing both of my Krav and their Wildshape Brontosaurus.

After this setup, the enemy's Sadistic Mindflayers behaved as though they had Deathtouch. I blocked their incoming attacks from both Sadistic Mindflayers (one at 2/3, and the other at 4/2) with both of my Prodigal Students (both at 2/6) and my Prodigal Students died at end of turn. Next turn, I summoned a Ruin Crab and blocked an incoming 2 damage from the remaining Sadistic Mondflayer (2/1) and Ruin Crab died at end of turn (should've lived at 3/1).

Somehow, my creatures were either given Fleeting at the end of opponent's turn, or his were given some form of Deathtouch that only took affect at turn-end.

Hopefully this helps recreate the bug.
Thank you for the great game!