If a falling block is suspended in the air (still falling) and you change it's blockstate manually using /data, it will not change the appearance of said block until it hits the ground.
To recreate this, you need to summon a falling block, preferably with {NoGravity:1b}, then you must change it's blockstate using /data .
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We're tracking this issue in MC-72849, so this ticket is being resolved and linked as a Duplicate.
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