I think that's at least half right. Many of the dynamic hapd points are unpainted, yet some are painted. And other areas of the mechs also appear unpainted.I think I know what you are referring to.
I have noticed that areas of the model that deal with dynamic hardpoints seem to not have a color or camo applied.
Basically where the missile tubes or laser lenses appear when populated on the mech.
This may be a limitation derived from how the model is built. It might be those areas because they dynamically change with load out might not be treated the same as a surface that stays more static with load out.
Just a guess on my part.