Great Tusk - Land Collapse

I was playing with the Great Tusk Mill deck and used the Ancestral Supporter card, which resulted in me having no cards left in my deck. It was my turn, so I used the Land Collapse attack of my Great Tusk, but it only discarded 1 card from my opponent's deck. My opponent had only 4 cards left before the attack, and I lost the game due to this bug. They should have discarded 4 cards, but only 1 was discarded.

Best Regards.