A hopper that is not full, but has items in each of it's slots, will attempt to pull bone meal from a composter. Since the hopper is already 'full,' the bone meal is voided. I've attached a video that demonstrates the bug.
MC-217946
Hopper Can Void Bone Meal From Composter
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Looks like this has already been reported. I had searched for this issue a long time ago, but did not report until now. Forgot to look for an existing ticket this time...
Possible original ticket:
MC-175090