When you killed a zombie and they have loot on them you HAVE TO put it in your hand first to be able to put it in your inventory.

So, it is currently NOT POSSIBLE to:
*use the assigned button (inventory) to put things there.
*put the loot on the ground/floor and put it in your inventory then.

The only possible way to get loot from zombies is like said before: you have to put it in your hands first.

It happens to everybody on our server and i have reproduced it on several other servers too.