for an exceedingly rare and difficult boss monster to have drop failure is INEXCUSABLE and i am assuming this is a bug.
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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.