TNT falls through blocks

*Additional information by [~Auldrick]:*

Primed TNT at Y=64 will fall to Y=63, even if there is a solid block below it. It will then continue to fall until it encounters a solid surface below Y=63.

Steps to reproduce:

1. Place a TNT block on or above a surface at Y=64.

2. Prime the TNT.

Expected results:
The TNT falls to the surface, stops, and explodes there.

Actual results:
The TNT falls through the Y=64 level to Y=63. It then continues falling until stopped by a solid surface below it.

Additional information
MCPE-43744 describes a similar issue that a player who stands on a chest at Y=63 will fall through it. This suggests that there is an issue with entity hit box detections at Y=64.