Hoppers put items in restricted slots

I made a charging block (kinda like a furnace) that charges batteries. Here's what it looks like:

The slots are not restricted except for slot #0, which has a restriction to batteries:

But if you use a hopper to feed coal into the block, the coal goes into the restricted slot #0. How do I fix this without having to disable hoppers from inserting into slot #0? Or is this a bug a developer will have to fix?

(edit)

I think I know what the problem is. The hopper puts the items inside the block's inventory, where there is no restrictions. It is only in the GUI of the block that has the restriction.