What is the downward acceleration of a projectile that is released from a missile accelerating upward at 9.8 m/s2 ?

The downward acceleration of the projectile that is released from the missile upward at 9.8 m / s² is equal to the magnitude of the acceleration due to gravity which is 9.8 m / s². When the object moves upward with acceleration, the acceleration due to gravity always acts on it, accounting for the force of gravitation experienced by the object.
Therefore as long as the object is under the influence of the gravitation force of Earth, it will continue to experience the acceleration due to gravity, no matter if the object moves upwards or downwards.