I previously reported this problem but used a modified version of Minecraft so it was resolved as inaccurate, I've recreated the problem in vanilla Minecraft 1.13.2:

When kelp is broken off by a piston and the base kelp is left to continue to grow, the age of the kelp no longer increases by 1 with each growth.  The age displayed in the debug screen seems to be random with each growth after being broken by a piston.  To recreate this, place a piece of kelp down and notice how the age goes up by 1 with each growth. Then use a piston to break the kelp off, but leaving the bottom kelp to regrow.  Take note of the new age of the kelp, and how it no longer increases by 1 with each new growth.  It seems to be randomly generating a new age. As you can see in the attachments, the kelp that is one block tall has an age of 15, and after it grew, its new age was 13.   The kelp should continue to age by one after breaking a piece off with a piston.