A super missile counts as two projectiles. When the graphic projectile dies, it deletes all super missiles. ALL.
So, if you have two super missiles on screen, the moment either one dies, the other one dies as well (give or take a frame).
This is really stupid, especially considering that the routine to delete the multiple super missiles is less efficient than using the graphic projectile's link to the super missile.
So I made use of just that. This code (should) allow you to fire multiple super missiles without worrying about them being deleted when they shouldn't be. Also, this fix exists entirely within original code; no extra space is used, so unless you tweaked this specific code, it should run fine on any hack.