Instabilis seeds cannot be grown in the way described in the book. The seed destruction mechanism destroys the outer seeds, which causes a chain reaction, and all the seeds are destroyed in the direction of the center.

(Unique Crops version 0.2.1)
They appear to be working correctly for me:

Are you actually using shears on the grown crop (as the book tells you), or are you simply punching it?
The chain-reaction of seed destruction is intended if you don't use shears.