All missiles, even missiles with primary documentation regarding extremely low engagement altitudes such as CAMM have MP and are all affected equally and set to 60m
The thing about shooting down onto the target is that the missile is still being multipathed, but the target is actually in the way and still gets hit. But they can still miss if the angle is right. To this end, it is possible to loft a missile from low alt, at someone in multipath and get the missile to still hit.
Though it is time for MP to be set on a per missile basis and AMRAAM would at worst be set to 40m, but potentially lower