That upper pipe looks like it has a flare to hold a donut gasket. I'd bet it should have a small hard donut type exhaust gasket. Those flange brackets aren't connected to the pipe but hit a flare stop on the ends of each pipe and clamp the pipe ends together. A gasket between the pipe ends is used to seal them. A flat gasket between the flanges probably will not seal as the pipe ends have to be sealed, not the flange brackets. Have you checked on top of the bottom pipe to see if a gasket isn't setting on there that you can't see from underneath? You should be able to pick up a donut gasket at any small engine repair shop or of course Amazon. This is just from looking at that pic so I could be wrong
You really need to get up there and get close to see how they seal.