Bug for Minecraft Falling Block

I was dealing with Minecraft command blocks.
For the work I want, I want to make sure that the blocks fall.
I was dealing with it. There was no problem with the y-coordinates until ~-1.5.
No, but the moment the y-coordinate is ~-1.51,
A problem has occurred. It's a block that falls right there.
The block below one column by reference has disappeared.
It literally disappeared. And I kept installing blocks under the block,
It didn't work. The command syntax is...
This is:

summon minecraft:falling_block ~ ~-1.51 {BlockState:{Name:"minecraft:grass_block"},NoGravity:1b or 0b(true or false)}

Please understand that there may be errors because it was translated through a translator.