Hey there, i’m Not on PC right now, but There is a Node in your pawn spawndefaultcontroller. Call this and Feed the Output Into the posess Node. Why this happens? Because when you Set it in gamemmode as Default pawn, you automatically posess it when IT IS created on gamestart with your playercontroller, Not the ai. And a pawn can only be posessed by one Controller at a time.
If it still does Not Work Post how you ve Setup ai