Items in your inventory will stack in the latest PS Clients [0.3.015b] but only under the following conditions:
1. Items that were stacked in an inventory slot of your character in PS Client [0.3.014] and have not been moved out of your inventory, will still be stacked in a slot.
2. Items that are of the same type (i.e. iron ores) AND of the same Quality will stack in the same slot of your inventory.
Click-drag the item -for example a Dagger- up to the Eye Icon [Examine] and you'll see something like - Quality: 23.6/50
If you later loot another Dagger, and it's Quality is also 23.6/50 -it will join the Dagger you have already in the same slot.
Then you loot a third dagger and it's is Quality: 27.2/50 -it will go into a slot of its own
You can equip just that third Dagger, and go fight a spawn that isn't likely to kill you, and fight a few rounds... you can wear it down until it's quality is the same as the other two daggers, and then all three will stack in one slot.

Is this a bug? I don't think it is... this is too well thought out, I think it is intentional.