Guardian of the Great Door was in my starting hand and three subsequent mulligans in my last match, presumably because the game thinks its low mana cost of 2 makes it a good card to have in your opening hand.

But since it has an additional casting cost of tapping 4 permanents, it is highly unlikely to actually be of use in your starting hand.

I think this card should be counted as at least 6 mana cost for the starting hand generator, because otherwise it skews your opening hand.