This is particularly prolific in multiplayer, where you will engage the same Nemesis multiple times, and each time more offscreen enemies will be pulled in. Sometimes the delay can be upwards of 30s as they march over and around corners. They can also often offscreen you and killed you instantly - usually you run into nasty mobs and have the chance to fight on your terms, but at high corruptions having long-range enemies with no telegraphs attack you proactively from offscreen is a recipe for death, particularly while looking over prophecy/Nemesis loot drops (and therefore losing it all).
I can reproduce this every single time, no exceptions. Remove the relic so no totem? They still come. Multiplayer with 2 friends? 3 Nemesis fights, 3 waves of offscreen enemies come. Clear three screens in every direction? They’ll come from four screens away.