Ok, so did a bit more digging, and found I had not seen the initial simulation button (before starting the game). When doing like this it is working fine. I was just looking at finishing the game simulating (End Game) option when game had already started.
Were there any updates on this? I just started playing from GamePass on XBOX and having the same issue. Assistant will not do any subs under any circumstance... kinda pointless to even prep a team and subs