The Mana World Forums

Maybe is not a bug but everything in pickup list that means to be ignored is going to be picked up automatically as any other item.

this is normal; automatic pickup is enabled server-side on the hard server so items are added automatically to your inventory and this means the client is not actually picking up anything

So I guess that's better put an item to ignore in a drop slot in order to free inventory from those annoying knives when harvesting red slimes. Or leave the cave, go to Hurnscald and make a knife drop event...