Hello, I have come across this issue when trying to put a block on the ground whilst it staying as an entity, and it didnt work.
Commands used:
/summon minecraft:falling_block ~ ~ ~ {BlockState:
{Name:"minecraft:stone",NoGravity:1b,Invulnerable:1b}
,Time:574981657349}
/summon minecraft:falling_block ~ ~ ~ {BlockState:
{Name:"minecraft:stone",NoGravity:1,Invulnerable:1b}
,Time:574981657349}
Because you put it in the BlockState compound, rather than the root...
Fix up your command syntax before you decide it must be a game bug.