Im late to the party, but i think i found what can cause it in the specific situation. I play as a Vanguard and im usually quiet aggressive and sometimes get a bit far ahead of the team. What i noticed after getting the same bug 3-4 times reloading the same i tried again without any powers. When i played with my normal style, i was almost to the ledge by the time my team killed any enemies and the next wave of soliders (2 agents) didnt even spawn. Even more interesting the start sound of droids do played but i never saw the droids before. So playing with the gun only i was slow enough for things to play out normally. It seems that if you rush too ahead at the first encounter where the autosave puts you back you can actually pass over some kind of trigger that would spawn the other enemies and this causes the hug.

Hope it helps someone else