I searched through these forums for "MIRV bug" and only found the one about customized Klingon ships with a mixture of MIRVs and other missile racks on a Dynaverse server. Sometimes your normal missiles would do less than expected damage. Well, I did some tests and found that if you skip spacedock, going straight into battle, and a MIRV rack occupies the highest weapon slot among missile racks, the DroC, for example, will do just 2 points of damage and only have 2 hitpoints! Weird. If you fiddle with spacedock, the correct data is written to memory (at least in skirmish mode).
Take a look at the damage estimator for DroM. It seems to always display 0-2 (2). You'll know your regular missiles are bugged if the estimator shows that info for Type-I or Type-IV ammo in a DroA through DroG rack.
Looking at the code, I see no function that assigns an alternate warhead to the bus vehicle or the submunitions. The MIRV bus is effectively a single 2-hitpoint, 2-explosive strength submunition with separate (but identical by default) speed values. When it blooms, the bus vehicle is destroyed and six little missiles are created (analogous to a scatterpack). If the bus vehicle encounters its target (as in an anchor) before it can bloom, it delivers 2 points of damage.
So a ship with, let's say, 4 fast MIRVs is really toothless (missile-wise) if you can manage to get to range 3 or less. They'll collectively do 8 damage per turn if they can't bloom. The deployment time can be reduced to a millisecond instead of 3.0 seconds, however. On the other hand, it can be increased to 30 seconds such that the MIRV has 4 turns of combined endurance and can strike from 128.0k away -- that's similar to off-map bombardment.
I'll make more MIRV variables customizable as I find them. And people can playtest them.