Sorry, it was pretty rude not to actually explain what it was. Level 4, picked up the Sapphire Key, the combat section started and when all enemies were slaughtered the portals never disappeared.
Hard to say what it was, but I think it was the third wave, killing an enemy either the frame it appeared or maybe the frame before by pre-empting a spawn with a fireball AOE/ice crit. Not an easy one to reproduce, I'm afraid. Windows whatever-the-latest-one is on a pretty decent rig, for what it matters, I don't think it was performance related. If I had to guess, the enemies have to fully spawn and run a frame before their death contributes to the death-count necessary for the wave to count as completed?