Anticheat senses the block over the repeater - It determines which side of the block you're trying to edit the block from, and if the block is a solid, non-transparent block (in this case, the dirt block above the repeater), then it'll prevent the editing packet. In order to save processing power, it simply checks for a solid/non-solid block and doesn't check dimensions for items such as repeaters and buttons. If you right-click it from a side that has an air block, though, it should work fine.
Hope this helped :p
