As of v1.2 on a Windows 10 PC, I've noticed that the set quantity option for item stacks is randomly buggy.
It seems to work correctly about 80% of the time, but if I put down 10 potions or herbs and set quantity on each of them to 9, when I play the module, 8 of them will correctly have 9 copies in a stack, 1 of them will only show 1, and one of them will have 3... It's very bizarre. (if I go back into editor the quantities are still correct. It's just when I play the game, the player gets the wrong amount.)
Note that I'm using potions and herbs here as an example. I've run into the same issue with rings and other items, but I'm not sure if that's because those items aren't designed to stack. But potions, herbs, and ammo, should always be stackable.