The motion of a TNT entitie is never set to 0 nomatter how slow it is.
There are two causes of TNT "micromotions"

-> When a tnt entity is created, its motion in one direction can have a tiny value

-> the motion of tnt gets multiplied by a specific value each tick
making the motion of the tnt smaller in each and every game tick.

As long as it does not collide with a wall , its motion will never reach "0"
TNT Entities with motions near to 0 are unpredictable and cause problems in complex kanonen.

 the players motion however, will be set to 0 once it reaches a threshold