Keyboard key "Q" bugged

Confirmed, but with qualification. The Q key actually works, but it works a little differently than it does when you aren't using a UI panel, and there's some confusion caused by the fact that sometimes you aren't getting the visual feedback you need to understand what's happening.

When you open your inventory, the top left slot in the inventory is initially selected and highlighted in green. You can then use either the keyboard arrow keys or the mouse cursor to select either a different slot or one of the buttons at the top of the panel. The UI switches between keyboard and mouse selection depending on whether you last pressed a key or moved the mouse.

When you're using the keyboard, a slot or button is always selected and highlighted. But as soon as you move the mouse, the highlight starts following the mouse cursor instead. As long as the mouse cursor is pointing to a slot or button, the highlight moves there just like it would if you had pressed an arrow key. But when the mouse isn't pointing to a slot or button, the highlight is hidden to signal that nothing is selected.

I assume that your custom is to use the mouse (as I think most of us do), and that because your mouse starts out outside the UI panel the initial highlight gets hidden so quickly you don't even notice it. The lack of visual feedback probably leads you to expect that the Q key will drop whatever was last selected in your hotbar. Unfortunately, that isn't how it works. If you point the mouse cursor to various non-empty slots and press Q, you'll see it drops an item from whichever slot is highlighted at the moment.

This is probably how the designers intended it to work. However, the developers may want to consider whether some kind of additional feedback would be appropriate here, because it is subject to misunderstanding.

Confirmed in 1.2.13.5 (1.2.13 Beta Build).