(entity instanceof PrimedTnt ? entity.getY() : entity.getEyeY())

in 1.8, the tnt and sand height was changed to 0.0 and 0.833 blocks respectively. The result of this is that fallingBlock (sand, gravel, anvils, concrete powder...) and tnt entities no longer travel along the same trajectory when exposed to an explosion on the X or Z axys. 

How to fix it?

Adding the fallingBlock entity to this special case would allow sand and tnt entities to follow the same path when propelled by an explosion in multiple dimensions (x, y and z)(they already get affected the same way when the explosion is directly underneath the the projectile, being fallingBlock or tnt).

Why even bother fixing it?

A lot of people like myself who make tnt cannons would love to see this, because we often shoot both tnt and sand entities out, which is very inefficient now. It will not noticably affect other players, because they usually don't shoot sand entities with a lot of tnt in "normal' gameplay.

In conclusion, if it would be possible to make it so explosions affect both tnt and sand entities from their actual Y position instead of their eyeHeight, it would solve a lot of problems for us, it would not impact performance, and it will not break other current contraptions.

NOTE: the issue will not be fixed by making tnt get affected from its eyeHeight too; it would just propell itsself up then because the explosion takes places at the top of the bottom pixel (6.25% the size of an entity), the only fix is making both fallingBlock and tnt affected from the bottom.