I've seen the invisible foundations before as well in multiplayer only. I verified that another player can see the foundation blocks. I can not see them but I can walk on them though since my client does not see the foundation it tries to make me fall, but the host knows it's there and constantly tries to correct my position. This results is a jumpy movement across the foundation. It is solved by another player removing and replacing that piece.
I believe this is just having to do with network latency / lost packets.