Since you are using Genesis.
An interesting feature added to UE4 is you can import and update the character rig that contains unique children bones and will create a series of phantom bones, greyed out as in you pic, that will accept animation keys if they are present in the animation data or ignore them if not.
This allows you to make use of the same base character rig, Genesis 3 for example, but account for added asset bones in Daz studio that accounts for unique asset requirements.
In context this is more of a mater of understanding what Daz Studio does and not so much a issue with Unreal 4 as it’s doing what it’s told to do. There are a few reasons as to adding phantom bones but the most common is not “Fit To > Genesis 3” as to added clothing assets so when exported is done as a unique element to the character, and create the phantom bones, when fitting it to the G3 framework will combine the clothing asset as part of the export.
In this configuration best practice is to make each character unique as to fit to finish needs as you will run into compounding problems. IE the more you add the more messed up it will become. Good example is the Paragon assets which use the same basic Epic rig but are unique as to animation and clothing assets.