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:
- Place a TNT block on or above a surface at Y=64.
- Prime the TNT.
The TNT falls to the surface, stops, and explodes there.
The TNT falls through the Y=64 level to Y=63. It then continues falling until stopped by a solid surface below it.
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.