Thank you for your answer.
Made me dig deeper into how I created the object from C4d with Riptide plugin for export, and realised that some materials were missing from the start in those files, but were replaced by the grey default diffuse color when rendering on my computer. When gathering the project though, the missing materials are actually not detected by terragen, trigger a warning, and then appeared black in the render instead of grey on my computer, which was misleading for me and made me think it was loaded correctly.
My bad,
Thanks a lot!
M