Okidoke, so I've been dabbling with creating my own inventory system. Let me start off by saying my logic is probably really gross and can be simplified, but everything works... unless I move the camera to another part of the scene...
Now I know what you're thinking, I didn't compensate for the camera with the drawing of my menu... but I did! The inventory system spawns in the center of the screen, the slots and icons spawn in the correct position, visually.
However, the icons are only clickable in the inventory if I stay in the original camera position at 0,0. If I move the player down, and say move the camera 50 pixels down, the icons and inventory spawn exactly where they need, but I have to click in an invisible space 50 pixels above the icon to use the item. It's like the collider is staying at the original X,Y with the camera, but the icon (actor) is moving with the inventory as intended.
For this prototype, I applied the event to use the icon directly to the actor, not a behavior, but as an event. Just as simple as "If clicked and health not full, give a little health", and the same for the magic. All of them work as long as I don't move the camera.
Here's a quick Gif Example. First one is working in the original X,Y camera of 0,0. The second is an example of what happens if I move down and try to click on the menu item.