In my kelp farm the kelp top is harvested by pistons. But sometimes the remaining kelp block doesnt turn into a top one again so the plant stops growing. After the world is saved and reopened the highest kelp is a top one again. The F3 screen shows that the bug is not only visual.

Edit: I noticed that the player needs to be a certain distance away from the event to recreate that bug. I provided instruction how to reproduce.