The InventoryMoveItemEvent is not so consistent if you want to know the contents in the container. 

No mater what It set amount on first item it try to move to 1 (depending on settings for hopperAmount ofc). even inventory get modified of this event no mater if you cancel event or not.

So should be nice to have access at least a clone of the original amount. Only option right now is ether get inventory 1 tick after event get trigged or self use nms to fix it.

Print the inventory it moved from and if you only have 1 stack it will report 1 item no mater amount you added.