Hello Vincent.
I have recently tried your suggestion of using the udatasmith format to import my model to twinmotion. Unfortunately this hasn't helped at all! Once I went back and edited the model and updated the import, I lost materials again. This is very frustrating!
In the import menu, I selected the option to 'use scene material' with enable substitutions unchecked. Please see the attached video showing clearly what happens when doing something as simple as moving a window in a wall. Can you please inform me if this is how it is intended to work or if this is a bug? 
Is there something I am missing or doing wrong either at import stage or at modelling stage that is causing this to happen? I have tried various different settings and still no joy!
Daniel.