Observer stunts kelp growth

I have an observer detecting kelp growth and it is meant to trigger when kelp gets to a height of 3. I have observed that the rest of the kelp regularly grows to full height, but the kelp in front of the observer remains at level 2. It doesn't do it always, but it does it often enough that I am convinced it is not a coincidence. (I have a third occurrence on video that I haven't attached; I have a manual flying-machine trigger built on my farm because the kelp in front of the observer so reliably doesn't grow).