The prison ghost is not a naturally existing Yin entity, but man-made.

Perhaps it originally contained two types of jade. The divine power annihilated the "spirit" of the prison ghost, and Feng Yaya dispersed the "body" of the prison ghost, resulting in each only obtaining half of the jade.

The method of annihilating the prison ghost is likely not the orthodox way of collecting soul jade and warm jade.

But it doesn't matter, there are countless prison ghosts in this space, enough to squander!