Sand blocks dropped underwater on fence go into the Twilight Zone

While attempting to build out, on top of a shipwreck in survival - I began dropping sand blocks to clear the water out for air. However, some blocks simply dropped endlessly into the wooden spruce blocks below. Several of these became frozen in the wall, between block boundaries. One cannot remove or even modify this ghost block of sand. It also appears to have no virtual mass and while the texture is rendered as matter, it is simply not there.

Please see attached.