The reason:
Meta humans have certain bones cut at LOD levels, simplifying the skeleton to improve performance.
The solution:
Match the LOD settings of your custom clothing to the Metahuman.
The how:
Select MetaHuman_LODSettings on your clothing. Wait for the mesh to ready. Set the number of LOD’s to 8. Test by zooming in and out of the viewport of 3rd person character or any animated character wearing the clothing to see if the bones stick.
I haven’t been able to get Metahuman_Clothing_LODSettings to work. only works with 8 levels of LOD. This could be because its the max levels set for the LODsync component, or because its the number of levels on the Face mesh.
I have literally spent days trying to fix this problem and have gone insane.