Falling blocks dont update visually.

If a falling block gets its ID updated, it changes its loot table, but not its visual appearence.

You can check it out by running /data modify... on a falling block of your like.

Furthermore, you can make it drop to see that indeed, it changed to another block ID.

Note that this bug have been in the game from before the flattening, It is present in 1.9, 1.10, 1.11 and 1.12 just need to change the command with /entity

Video showing the bug: https://youtu.be/uDPBu0IYK-Q