Weird indeed! A shot in the dark: If you created the child BPs of BP_MasterWeapon before populating the light attacks array in the BP_MasterWeapon, but after creating said array, you might want to recompile & resave the child BPs. And verify that they have the populated version of the array.
I could be wrong, but if you created the light weapons array, left it empty, and instanced child BPs, it could have stayed empty on the children when you populated it in the parent. Maybe? At least that’s how it would work for a C++ class with a BP child.
EDIT: I also notice that the array is “Instance Editable” + “Expose on spawn”, if you left the pin empty on spawn, it could lead to an empty array that overrides the filled default value? Possibly. Either way, if you populate the array from the BP defaults, there is no need to expose it on spawn or even make it instance editable.