There is a bug that causes boss packs to not spawn if the assassin lays any traps on the ground after entering an area!
Shadow Warrior, Shadow Master, Wake of Inferno, Wake of Fire and Death Sentry all increment the boss pack counter by one if they are cast. For all the traps it does not matter whether the Assassin or a Shadow summoned it.
its a not very known bug and is not very well documented. The LoD expansion was rushed and created several bugs mostly with the assassin and some with the druid. Some people guess the game tags the created traps or shadows as “unique monsters” and thereby decrease map unique monster packs.
Note that i do not have a source on this and googling does only give some questionable results. Yet several of the big D2 streamers and content creators refers to this bug. If someone have definite data that shows how much this bug impact the game please share.
Does the D2R team know about it? Are they working on but unable to fix because of old engine? Have already fixed? Have they looked into and the bug and it does not exist?
Older thread on blizz forum:
thanks to may13 and others for the previous links and some copied text.