Hello all,
I am just building a small demo for you. Having learned about z-fighting, I cut holes where paintings and stairs are. Trying to keep the model small, I simplified it a lot:
There is a large ngon where the stairs go, but the uv map looks good and Blender does not have a rendering issue:
But Defold has:
I suppose it has to do with triangulation when exporting with Collada.
Anyway, 3d-Defolders, beware of overly daring ngons. 