I don't believe there is a fix for that. All the mortar's I have seen fired, peel the aluminum off the cement. perhaps if the hole is sealed after filling and solidifying??? I suspect air entering the fill hole and if turned upside down, the gases enter the fill hole and allow rupture.
The guy that has one here just points out the aluminum streamers in the air after firing. He doesn't try to recover the "Shot" for another go.