The stack button is supposed merge things from your inventory into the chest. It will look at the objects in the chest and then at the objects in your inventory. Based on what is already in the chest the game will pull anything from your inventory that matches into the chest. So if you have dark wood in the chest already, and you click the stack button, the game will pull any dark wood from your inventory and try to place it in the chest.
As for the bug, the place all button now actually places everything into the chest. The stack button has the old functionality of only placing like items. If you have any sort of screenshot of this I’d be happy to see it. I’ll try to break it myself but so far no one else has reported something like this.