Nah, it's fine as sponge blocks as yeah that's the disadvantage of the Sponge Island :) Howerver, if you move a little bit with a block bridge you can place it, or you can simply break the sponges and place it back.
Okay this is a small bug made by our newest Anti-hack plugin we have in PvP. I THINK I fixed this bug, but still need testing. So if this happens again please comment here and I'll check again what may be causing it :)