When you shift click on TOP of a hopper with a button a button is placed. That doesn't work with all other sides. Shift clicking on top of a hopper with a button should not place the button on the hopper.
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Not a bug - redstone dust can also be placed on a hopper in this way, which makes it easy to lock and unlock hoppers with redstone.
Having a button on top of a hopper allows you to lock it temporarily, which might be useful, idk.