When summoning a FallingSand entity with motion tags, the block disappears when flying. You see the block when it lands on the ground. When you switch to 1.7.4 it works perfectly fine. Sorry for the bad quality of the video.
Squid
MC-49593
Invisible FallingSand
Resolved
Duplicate
0
Squid
(Unassigned)
Unconfirmed
Minecraft 14w08a
Retrieved
The same happened to me with any tag. I cannot see the entity until it solidifies.