Why do faces appear transparent when exported to a game engine? I created a low poly tree, and it's what I see in the editor:



But, in my 3D game engine (Godot Engine), this is how the tree looks:



(On the left, this is an icosphere created with Blender).

I don't know what part I missed, or what I done wrong for having this result with the 3D engine.

I started with a simple cube, cutting it and extruded it for this result, and I created manually near all the faces (except for the trunk). Then I exported the object in the Wavefront format (.obj).

Is my method wrong? Need I to recreate the whole object? How can I fix it?
Did you try switching the normals?
If not, select the entire object whilst in Edit Mode. And press Crtl + N