Summary: When you summon a falling block, the game renders it the way you specified in the command to summon it. If you then modify the block id, the falling block still renders as the block it was before you modified it. For example: if you summon a falling stone block, and then change the block id to oak wood, the falling block will turn into oak wood when it lands but still render as stone while it is falling.

These are the commands used in the screenshot below:

1. summoning a falling block of stone:

summon minecraft:falling_block ~ ~2.7 ~ {NoGravity:1,BlockState:

,Time:1}

2. modifying the falling block to be a block of oak wood:

data modify entity @e[type=minecraft:falling_block,limit=1,sort=nearest] BlockState.Name set from entity @e[type=minecraft:item_frame,limit=1,sort=nearest] Item.id

3. the third command confirms that the falling block, which looks like a block of stone, is actually a block of oak wood:

tellraw @a {"nbt":"BlockState.Name","entity":"@e[type=minecraft:falling_block,limit=1,sort=nearest]"}