This fixes the target is 'nil' sort of errors people have been reporting lately with these hooks, particularly if they had multiple instances of these hooks or loaded a particular combo of addons at once. Turns out specifically it broke if you had both a generic kind and type-specific kind of one of these hooks!
Long story short, it was some support for "damagetype" being accidentally left out that lead to this. If you had a generic type of one of the three hooks, it mucked up the type-specific hooks such that the function args you receive are then all offset to the next arg slot!
MobjDamage: target = userdata: 13BAB0E8, inflictor = userdata: 13BAA548, source = userdata: 13BAA548, damage = 1, damagetype = nil
MobjDamage: target = nil, inflictor = userdata: 13BAB0E8, source = userdata: 13BAA548, damage = userdata: 13BAA548, damagetype = 1(The script I used to test this was luatest-mobjdamagebug.lua)