Probably not a bug per se, but I often feel like I'm missing things when I watch Spectator Mode.
Each turn, only the turn player's field updates. This means that, for example, if they used any sort of removal, a spectator won't see the effects of it until after the opponent's turn. Also, I can't be sure, but it seems like the statuses that, in normal play, have some sort of visible effect on the card (i.e. burrowing, becoming immortal, adrenaline) don't show up in Spectator Mode.