for an exceedingly rare and difficult boss monster to have drop failure is INEXCUSABLE and i am assuming this is a bug.
Linked issues
duplicates 1
Comments 3
i can reproduce this bug on a flat bedrock sheet with no area hazards and no other mobs. perhaps the nether star needs explosion resistance to mitigate the fact that it drops near a wither
This ticket has been marked as a duplicate of MC-4117 , in favor of the better description/information provided.
I know for a fact that it always drops. It might get destroyed by an explosion, lava or fire.