Floating kelp plant.

Steps to reproduce:
1. use /gamerule randomTickSpeed and set it to a high number.
2. place kelp and let it grow
3. randomly break it

And then you might get a floating kelp plant.

Edit: I have noticed it is easier to reproduce the higher the tickspeed is, I don't think it really has to do with the tickspeed, but with the growth of Kelp.