Even though the texture shows it is ready to be harvested, and it allows the composter to be emptied, it does not always give you bone meal.

A quick way to verify:

place a composter,

get 1-2 stacks of a compostable item (Tried with a stack of kelp)

fill the composter until you can harvest bone meal

repeat until out of stacks, keeping track of how many times the composter was emptied, and how many times it fails to generate bone meal.