To reproduce:

1.  Put the following command into a repeating command block, set to always active.  

   /kill @e[type=minecraft:falling_block]
   

1. Place a block that is able to fall where there is an air block below it. (May have to do it multiple times)

Outcome:

1.  The block will remain in the air and not fall.
2. When gamerule commandBlockOutput is set to true, it says in chat that the falling blocks are being killed even though there are no entities being killed.
3. Sometimes, there will be invisible blocks that can be stood on. The block isn't affected by updating the block next to it.

Note:

1. This was in a new world but this also worked on another world that has been updated many versions.
2. I had to lower the quality of the video in order to lower the file size so the debug menu is hard to read.
3. I did not test if this worked on a multiplayer or a LAN server, so I was running a singleplayer world that isn't open to LAN.
4. I only tested this with gravel.