Natural generated sand and gravel do not update

What I expected to happen was after I placed a block next to sand/gravel above air to fall down on the blocks beneath the sand/gravel block.

What actually happened was that when I placed a block next to the sand/gravel it did not fall down. Only when I placed a block under the sand/gravel block it did fall down.

Steps to Reproduce:
1. Generated a new world in snapshot 17w47a.
2. When generated I did see sand blocks above air like a cave.
3. Went into the creative menu to get a block that is not affected by gravity.
4. Placed the block that is not affected by gravity next to a sand block to update the sand
block.
5. The sand block did not update so it does not fall down.
6. Placed a block on top the sand block to update the sand block.
7. The sand block did not fall down.
8. Placed a block under the sand block to update the sand block.
9. Removed the block under the sand block.
10. The sand block fell down.
11. Other sand blocks did not get the update and did not fall down.
12. Tested the same steps in survival mode.
13. Only when a block is placed under the sand block it did update and fall down.