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
Linked issues
duplicates 1
Comments 5
Duplicate of MC-72849 which was later marked as "Won't Fix". See the duplicated link for further details.
Decided to reupload, since in 1.16 they fixed an old visual bug that was initially classified as Wont fix.
Please provide a screenshot of this occurring with the F3 debug screen enabled.