I think this is happening because cc3 characters usually have 2 neck bones while everything modeled on the Epic mannequin has only one neck bone. If you are retargeting existing animations onto the cc3 character than one neck bone will receive animation while the other will stay in default rest pose.
If this is indeed the problem then possible solutions would be either delete one of the bones and smooth out the weight paint or use the AnimInstance graph to copy rotations or otherwise smooth animation from one neck bone to the other so they both move in tandem.