Minions can be overwhelmed and I've seen them being overwhelmed lots of times. The only difference is that when a minion is about to be overwhelmed, there is no red highlight of his strength. If you had a game when a minion was not overwhelmed while it should be, post the game replay here explaining when did this happen (game replays of your latest 20 games can be found in Games History tab). Then either someone will find a reason why the minion was not overwhelmed or the (very specific, generally minions are overwhelmed when they should) bug will be found and MarcinS will fix it.