I created a curved tube with a cutout. I exported the file to various mesh formats including USDZ, OBJ and STL.
In all of these exported files there are gaps along the seam where the walls of the tube join the edge of the cutout.
It appears that the cause is that the cutout and the tube use different angular distances for tessellating the curve, and coincident vertices along the seam are not connected.
It appears that adjacent edges are not joined or welded together when they have different materials, and the gap is visible.
When adjacent edges have the same material the edges are joined together. The workaround then is to use the same material for edges that need to be joined. The material can then be changed in another 3D editor such as blender.