When a sticky piston is put on a 3 tick clock and positioned facing the block above a hopper, if the chunk unloads/loads at the right timing, it creates an invisible, unbreakable block that has a hitbox showing it halfway between 2 blocks (the 2 blocks that the sticky piston would push it between). I had the sticky piston pushing a piece of cobblestone, which has vanished and is replaced with the unremovable block. Note, i am able to put any block in front of the piston, however the piston will not extend given a charge (look at attached screenshots). I cannot place a block in the location that the piston would push another block into. In case you don't understand. In the screenshots, you can see the outline of an invisible block which we (its on a server) can't break. Of the 2 blocks it is on, you can place anything on the left, and cannot place anything on the right. You can walk through the block just fine. It should be important to note that there are two of these on top of one another (third picture shows the upper hitbox).
Edit 1: When it was created, there was a second hopper under what had been the block the cobblestone would be pushed on to. Again, cobble gone, new block there. I mined up the hopper (its in my hotbar) because i was trying to figure out whats wrong.

Edit 2: Thank you Matthew Hunter, right clicking it with an open hand did successfully make it go away, it only worked when we used an open hand. I will leave the issue open for now since this is still a bug, I had to reset a huge redstone system when block 36 got in the way.