When you have a model with multiple material ID’s and you drag/drop a material in the viewport on that model, the material you want to apply it to isn’t affected. Instead it applies the material to another ID slot rather than the one you released your mouse click on.
Ever since the new LOD/material ID naming changes it seems this is quite buggy
I hope my screenshot and explanation explains it well enough.
When you select the same object and apply the material in the detail editor it works fine. So the model itself or the export/import isn’t broken.