A couple of Infernals been acting this way. For each wave, when you’ve cleared out all of the mobs, instead of the loot pop, the timer disappears and nothing happens.I run around the map and nothing is there. Nothing on the minimap with Sobus detection. Until I hit a certain spot and my char slows down as I must have come within close quarter range of something. I attack randomly and then a kill must have happened and the wave end continues.
On the last wave and with the Fell bosses, it proceeds normally. Kinda weird. Like doing the 'tides in the early days when monsters would spawn within objects or the terrain and made it almost impossible to get the event done.
Posting this as this wasn’t happening before the recent 2.2.3 update.