After the Butcher spawns, you kill him and kill the three pack leaders, you get a buff that allows you to do very high damage to the next Gyfin pack leader, no matter their type. If you have 100% BSR and one shot the Gyfin pack leader during their tanky "spawn" animation, it prevents them from spawning. In the image below, only the small mobs from the Despair buff kept spawning. The next photo after the Despair buff ends shows an empty spot.
Edit: This is not during node war. This happened around 1 PM UTC+8:00
During Despair buff: