When a player places a block next to a fence and the corresponding PlayerInteractEvent is cancelled by a plugin, the neighboring fence block updates its state to 'connect' to the 'placed' block, although the block is not actually placed.

I assume that this change is client sided, since reconnecting to the server reverts the state of the fence block to no longer be connected.
Maybe the server is missing some update packets to revert any changes of neighboring blocks that the client predicts to normally take place.

There may be blocks other than fences that are affected by this as well.

The attached plugin simply cancels all right-click-block interactions that would usually place a dirt block. Trying to place a dirt block next to a fence will result in the fence 'connecting' to the space in which the dirt block would usually be placed.