this is just a logical behaviour. you don't change anything except position. So speed and previous acceleration will be stored.

if you want some good manual movement of physics, enable kinematic movement or change the entity type to character, then move it with pXent_setposition or pXent_move.

or as a dirty workaround: if you use pXent_setpositin, also use pXent_setvelocity to reset the speed of the entity.