Hah, I actually did search for "sign" but I didn't see this one. Sorry and thanks.
Added a new screenshot with the seed and coordinates. I don't think these are the same kelp as in the initial one, I was just flying around until I found a double.
Confirmed for 15w35b but only Dark Oak does that.
Hey, I made a video showing this bug. It's in the post itself as a youtube link, or here it is: https://www.youtube.com/watch?v=T6icK1QmDxQ
Like i said, this happens with any entity being ridden by a player, on water, ice or solid ground.