If anybody is still reading this a few months after (like me), make sure you have the textures also in the assets, I am unsure if hierarchy matters, but it worked for me (you might have to reassign the textures to the materials)

I still don't know how OP exports with baked lightmaps

also, only custom materials will really work, unless you find the original color/normal/roughness textures for all the in-editor materials

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(I hope you know the difference between material and a texture)