This bug was tested in Multiplayer when there only 1 player logged. I feel like reason for this bug is far distance between portals and crystal. If portals spawned far from portal mobs sometime spawn as invisuble/invulnerable untill they start attacking. To be sure, it happens if you rush from middle to left side (when you start event) and then in 5-10 seconds will go back on foot to the right. They will bump into you. For you it will look like mobs spawn, then huge space between and other mobs continue following. This doesn't happen if 2 players are in game and if they stay on both sides. Screeshots show my layout: First I go to the left straight away, then go to the right (first troops were already invisible) Then I went back to the left and as you can see there is a space between, where actually mobs that murder you. 6th Image is total distance between portals.