I'm fine with it, it's for the good of the Regnums.
This bug can be reproduced with any kind of stackable item. Here's a step-by-step example:
1. Stash a stack of items. Not all of them, keep at least 1 on your inventory. In my example, I will be storing 170 rocks from 177. I will have 7 in my inventory left.
2. Place the stashed stack in the Retrieve box. Should look like this:
3. Switch to the Store box and store, for example, 1 rock from your inventory.
The stash will show you the correct number of the stack of items you have stashed. That's what you want to see.
4. Switch to the Retrieve box and you will notice that the 170 items you had placed are still there. Feel free to place the stashed stack in the retrieve box now.
The result:
5. Proceed to go back to step 3 and repeat the process until you go out of rocks.
That's it, basically. If you press the Retrieve button the window closes and the number of items in the stash will remain correct. It's just a visual bug.