Using data merge or data modify to change the BlockState.Name or BlockState.Properties tag for a falling_block entity will not change how it looks, but it will still settle as the block it was modified to.
To replicate:
summon falling_block ~ ~ ~ {Time:1}
data modify entity @e[type=falling_block,limit=1,sort=nearest] {BlockState:{Name:"minecraft:stone"}}
It will continue to look like sand, but once it lands it will place a stone block
Thank you for your report!
We're tracking this issue as MC-72849, so this ticket is being resolved and linked as a duplicate.
That ticket has already been resolved as Won't Fix, which means this is considered a bug but won't be fixed. The description of that ticket or the comments might explain the rationale. Please do not leave a comment on the linked ticket.
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