This happened to me twice in back-to-back games. I was playing as Ashiok both times.

First time, I was facing Gideon. Life totals were both relatively high, minimum 15 cards left in each of our decks. On Gideon's first main phase (around turn 7), he resolves "Aura of Courage" and then wins. Doesn't even have to attack, doesn't have enough on the board for lethal, and I had blockers anyway, but it immediately calls it a win for him.

Second game I'm playing against Angrath, I have him down to 8 life and will likely win on the next turn. His time is running out for his turn, plays a creature, then just wins.

I have been having trouble with my internet today, not sure if it's relevant but I noticed delays during both games leading up to the automatic losses. No bugs or missed steps, but it would occasionally take 3+ seconds for a creature to enter the battlefield after I played it, or for fighting to happen after I assigned blockers.

This is an incredibly frustrating issue, especially late into a game when I'm close to winning. Please fix this ASAP.