Do not merge material slot
For glTF importing there's an option that prevents Blender from merging material slots that share identical materials. Could you also add this feature that maintains the original material slots for example after exporting as .fbx file? This is important for UE Editor.
but it doesn't merge material slots? I never added that
uemodel format itself doesn't merge slots, but when exporting as fbx slots pointing to same materials will merge. If you could add glTF-like feature that when importing uemodel to Blender, force material slot pointing to same material (i.e. with same name) to make an independent copy, and force the material also to make a copy, the exported model should maintain the slots as its origin.