Here's a tough one; I think I know what's happening but I'm not sure how to efficiently test it, so I was hoping one of you out there could help me out.
I was playing T50 against (I think) Donot's deck, which as of this writing involves a bunch of Mindgates and Hourglasses. The deck I was using packed a couple of steals as well, so before long the AI had a full field of permanents. Once that happened, I noticed that every time I'd put a permanent down, it would disappear (using the usual "stolen" animation) *without* a played card appearing under the opponent's hand. After a few turns of "wait what" running through my head, I played a Steal to take away something from the AI's side. Immediately he stole it back, with the usual Steal card appearing. After this point, my permanents stopped mysteriously disappearing.
[No screens, unfortunately, since I didn't have the foresight to take them while it was happening, and then once I formulated a hypothesis it wasn't happening anymore.]
What I think is happening: the AI says "I have a Steal! I'd better go take that shiny thing over there". The Steal effect takes the aforementioned shiny away from the player, finds no open permanent slots on the AI's field, and gives up, counting it as no play. Unfortunately, it doesn't seem to rewind the steal, so the permanents just disappear.
This seems to be really hard to test; I don't have a good idea of which AIs/FGs/T50 decks would be able to: (a) pack a bunch of Steals, and (b) fill up their field with permanents. Anybody ever heard of this before, or have a good way to test this? Thanks.