I’m not sure if this is a bug but it’s definitely an interesting situation. Normally when a rock is on a flat piece of terrain you can see the rock occupying the block above it. However when a rock is on a sloped terrain the rock only visible in the sloped rock cube yet the block above is still blocked and you can’t place a block there.
In this picture you can see what I am talking about and the block above I can’t actually place a block. I was confused at first until I noticed the rock there.