Essence monsters are dying but not dropping essence

On one of my maps this morning, I had two different essence monsters die without dropping anything. I double-checked this wasn't an item filter issue by turning off my filters entirely. Still no drops. This is unfortunate because it happened to monsters I had corrupted, and one of them was holding a T0 essence. Which means I was increasing my investment in the map only for it to backfire and for me to get nothing. This makes me extremely wary of trying to farm any more essences this league, and kinda bums me out on the league entirely.