When i import my skeletal mesh, which has every bone assigned, It says there are bones missing from the bind pose.
In Maya, it says the skeleton is fine and a normal IKHuman Rig.
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I also ran into this problem. What seemed to work (so far) is when importing into UE4 within the import menu there is a drop down box with a chick box TOAs
or something, I attached a pic to help you out. See if checking that box helps
I think this problem is in your design editor.
Be really sure if u use the correct bone to the correct object.