So I have this issue where I'm trying to import an M9 into studio from blender and some of the faces are disappearing. I've tried both FBX and OBJ files. The mesh looks completely fine in blender. Here's some images and a video of the issue.
The problem is when you export it from blender. Tick the ‘Backface Culling’, then go to your mesh in blender, and you will see parts that don’t look right (i.e. they are missing/semi-transparent). Click on the face(s) that you want to edit, and click Shift+N, this will then make the face ‘normal’. Hope this helps!