I have experienced this a lot on the Mac GoG version 1.0.5, running Mac OS 10.10.3 on a MacBook Pro (Retina, 13-inch, Mid 2014). I believe also on 1.0.6, but I will update to confirm this.

 

What happens is, after I've been playing a while and looting lots of items from lots of enemies, there will be a point at which the items start going directly to the stash rather than inventory. It is always fixed by restarting the whole game (not just reloading a save).

Just to confirm, you have:

 

1. Clicked/highlighted a character portrait and not the stash chest icon?
2. If above, the selected character has room in their personal inventory for the items?

I only ask because either case would explain items going to the stash and would be WAI.

I'm almost 100% sure of #1, but I'll double check. I don't even know how to have the stash selected. Whenever I want items to go to the stash, I just shift click them. I do remember seeing a character portrait (who has room) and his inventory above where it shows the lootable items. I then click the hand button to loot all at once. Besides, even if a character doesn't have room, this usually (when the bug isn't happening) results in the remaining items being given to other characters.

Hello.  If a character has full inventory and they are selected to loot it will send the items to the stash. This is intended.  It is also possible to click on the stash icon and have it be the default place to which items are placed.  Again, working as intended.  Thanks!