This is not a bug. The auto-mulligan system is designed to reshuffle your deck and redeal your hand if you don't draw a card with 0 cost once. If by the second time you do not draw a card with 0 cost, then it's basically too bad. Now, the chances of this occurring with this deck are extremely small (0.0076% probability with 28 cards), but it is not impossible, nor is it a bug.