If a chest contains money or an enemy has a psy card and you destroy the chest or enemy, but then another enemy picks up the loot, then the loot is effectively gone. When you kill the second enemy, he drops nothing.
There could be more items that when picked up by an enemy, simply disappear.
This not a bug exactly so much as a leftover behavior from Telepath Tactics! I could change it, but I feel that would reduce the number of interesting trade-offs players have to weigh when making moves. I'll give it some thought.