Merging Faces results in OBJ that has no face

I have made a very simple 3d object for use in Tabletop Simulator. Constructing the object results in many faces:

If I export this as an OBJ and import it, the object works just fine:

However, if I merge some of the faces together to reduce the number of vertices (merge 3 middle faces):

When I import the OBJ, the one big face is completely missing:

I'm not sure what is going on here. I would like to be able to merge each of the sides of the object I made to a single face (e.g. a top and bottom face). However, I've been unable to do so and have the object work in other programs. What am I doing wrong?

A face is defined as the area between either three (triangles), four (quadrangles) or more (ngons) vertices, with an edge on every side. These are often abbreviated to tris, quads & ngons.

Every 3D program will support tris and quads but not every program supports ngons. It would appear that tabletop simulator is one that does not support ngons, or doesn't support reading ngons from an obj file.

While you can keep using ngons when modelling in blender you will want to change them to tris or quads before or during export. You could use a remesh or triangulate modifier, or just tick the Triangulate Faces checkbox when you export to obj. The advantage of using a modifier is that you have some options to control how your model is divided and you can view the result before exporting.