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.
Comments 2
After reading your feedback, I removed the composter from where I had it, nestled between a workbench, a furnace and a one block high wall of grassy soil, moved it into a more open area, and repeated my trouble shooting. I can now confirm that the compost is generating bonemeal as designed, so that doesn't seem to be the issue. I now think that the bone meal was clipping into the blocks around the composter which is why I wasn't seeing it generate, and why I kept perceiving it as not generating bone meal.
Couldn't reproduce in 1.12.0.2. Are you sure the compost isn't going to another corner and you're just not noticing?