thanks for reporting the issue! Do you remamber the game number of that game, or against whom you played it? I checked your last 20 rated games (assuming you go by the same name in-game), but Guardian/Minion didn't came up together.
I tried to recreate the situation, and then it worked as it should: it protected me from discarding due to opponent's Minion.
2 things to remember:
1) Guardian needs to be in play to protect you from an attack, not in hand like a Moat.
2) If you can see the Guardian in play at the start of your cards, it will go away this turn and has protected you already on opponent's last turn, it will not do so on their next.
If you remember against whom you played and when, that might get as toward finding the game and look what actually happened.